im not as experienced as most people here but i have learned some things by reading. ive kept fish for years but i am almost as new to plants as you are.
first, welcome. you found a great community.
second, read read read....and then read some more.
check out: http://www.aquaticplantcentral.com/forumapc/new-planted-aquariums/8790-basics.html
before you start buying things i recommend getting all of the info. you can on every question you have. chances are that after reading, you will have more questions and then you need to search for those answers. luckily, you found this site. you can find lots of info. here.
it sounds like you are not only new to planted aquaria, but also to fish keeping.
i cannot stress enough how much research you should go do. but, let me say a few things.
plants need co2, fertilization (nutrients), and light.
cycling a tank: fish and plant wastes will turn into ammonia. there are beneficial bacteria that will grow in the tank. colonies of them. tons of them. these guys will consume the ammonia and turn it into nitrite. then, bacteria will grow by the tons and turn the nitrite into nitrate. this has to happen, you need it to happen, to have healthy fish. ammonia and nitrite are toxic to fish. nitrate is less toxic and wont harm the fish unless you let the levels get super high. you can control the nitrate levels by doing water changes. we will get to that in a bit.
this initial cycle can take weeks, sometimes up to 8 weeks but it is more typical for the cycle to complete in 4-6 weeks.
the nice thing is that you can put plants in right away. they like ammonia.
mechanical filtration: you will be best off with a canister filter. eheim makes nice ones, fluval is ok and if you want to save some money you can go with a rena xp line. for a 55G i would say a rena xp2 would work but a rena xp3 would be better. the best prices i have seen are at www.kensfish.com
. i think most would agree that eheim are very good filters...they all work, its up to you. the key is, go with canister type filters over hang on the back type if you can afford it.
substrate: you mention gravel. you can use an inert gravel, thats fine but you will need to feed the plants somehow. you can feed them by putting in fertilizer sticks or whatever in the gravel near the plants. you will also have to fertilize the water column. this can be done by liquids (seachem products for example) but those are pricey. you end up paying for a lot of water when you go with seachem. i say this because most of what is in the bottle is water...its a solution. the cheaper route is to go with dry ferts. im getting too much into ferts so ill save that for later. bottom line, when choosing a substrate, make sure you know what you are buying and what it can do for the plants.
my recommendation is going with ADA aquasoil. i just put the amazonia type in my tank and so far so good. link: http://www.adgshop.com/Substrate_System_s/1.htm
a couple nice things about amazonia. first, it is regarded by many as the best substrate available for growing plants. second, it softens the water and lowers the ph of the water which is good for alot of plants and is good for some really cool fish that you might want to keep. one downfall to this substrate is that it usually adds ammonia to the water. this is good for the plants but i say a downfall because it might add time that you must wait to add fish. remember, ammonia is toxic to the fishes.
this reminds me, you should go do some reading on water parameters. learn about ph, gh, kh, and more.
lighting: you will hear all kinda of suggestions for lighting, there are many good lights out there. lots say to go with over 2 watts per gallon for example, this really depends on the type of plants you want to keep. also, the type of lights, the type of reflectors, the distance from the light source to the plants will impact what kind of light and how much of it you need. i would say go for lights in the 5000K to 10000K range...you will have to read to find out what that means. there are different types of lighting out there, id say T5HO is a good choice.
my recommendation would be giesemann 6000K midday bulbs and for your tank i would get the 54 watt, 48" long types. you could get away with 2 of them but you might want to use 4 of them for high/intense lighting. these bulbs are wonderful and they can be bought with very good reflectors.
I bought mine from www.reefgeek.com
i bought the ballast, the bulbs, the reflectors and then put it all together. its not hard but if you are not into DIY projects then you might want to find a completed fixture but you will end up paying more money. if you need assistance in picking out the right equipment, you can always give the guys at reefgeek a call and they will help you pick out the compatible parts.
co2: for a 55G tank you should go with compressed co2. DIY will work but it can be tricky so if you can afford the equipment, a co2 system should be high on your list. this system includes a co2 tank (10 pound would be sufficient), a regulator, a needle valve, a reactor, co2 tubing and thats about it. you can get fancy with solenoids and ph controllers but you dont need it, its a luxury.
co2 is pretty important in a planted tank. you can go with a non-co2 tank if you want but that will limit the plants that you can grow. also, with high lighting and no co2, i hear that algae can become a serious issue but i dont know much on that. i think that if you want a non-co2 tank, just go with less light and you will be ok but again, there are going to be plants that you wont be able to keep.
fertilizers: i cant give much info because i havent used many ferts yet as my tank is new and im only dosing potassium at the moment. there are others around here that can help you and you can read all day on this subject. there is a nice fertilizer section on this forum. one thing ill mention, as i said above...going with dry ferts is most economical.
you will have to choose if you like the EI method or the PPS method or some other thing you may come up with on your own.
Water changes: DO THEM! depending on your water conditions and how much fertilizer you are putting in the water, and well, depending on alot of things, you will have to do some sort of water change schedule. given the decisions you make on how this tank will be set up and what you put in it and what you put in the water will dictate this schedule.
when I only kept fish, i would change 15% two times a week. i could get by doing 30% once a week or doing 50% once a week or whatever. the key is that i was doing them. if you try to only do it once a month then chances are the tank will get nasty. with plants and fertilization, it can be different so you can figure that out later.
plants: again im new but ill pass on the advice i was given. start out with lots of fast growing stem plants. plant heavily, like fill the tank with stem plants about 1 inch apart from each other. give them light, a shortened photoperiod (like 3-4 hours a day and then slowly over a couple months bring the lighting up to 8-10 hours or whatever you like). you are going to do your best to battle algae in the beginning. the fast growing stemplants will help you out on this. after some time, you will learn how to care for the plants and fish and you can trade some of the stem plants out for other plants that you may want to have in your tank.
be patient, it will all work out if you do it right.
ok, so there is much more to be covered but you will have to get that info. from other people and by doing your own research.
so a little recap:
figure out what sort of plants you want and what kind of fish you might want to keep eventually.
use this info to decide on lighting, filtration, substrate, co2 if any, fertilizers, water parameters you want to maintain to grow the plants and fish.
buy all that stuff, then buy the plants and have some fun.
keep an eye on this cycling thing i mentioned. when it is deemed safe, go ahead and add some fish. dont put too many in at once. add little by little...after some reading you should know what i mean.
again have fun and welcome to this wonderful hobby.....errr....obsession?