First, read http://www.aquaticplantcentral.com/...ums/14684-new-tank-setup-guide-parts-1-a.html and http://www.aquaticplantcentral.com/forumapc/new-planted-aquariums/8790-basics.html, then see if you have questions.
I second both suggestions given here. I just got the substrate and im pretty happy with the looks! very like to ADA AS appearance. only time will tell on performance but i have not read anything bad.As for the substrate, "aquariumplants.com's own substrate" has really worked out for me. It costs about $60 to $70 (shipped) for a 5gallon bucket of the stuff and it covers a 55g to 3 inches. I would defenitely recommend it. Don't forget to rinse it really well before putting it in the tank.
For other suggestions I 2nd Hoppy's reply and read through the links. Figure out exactly what kind of tank you wanna set up then ask specific questions. There is just waaayyyy toooooo much info out there.