i'm far from an expert, and i only have a 10 gal and 1 gal, but i've been doing a bunch of research lately, so i'll try to help you on what i know :)
i suggest going to theplantedtank.net and looking around. join, and you can go on the sales thread and get really cheap, or free plants! :D
What are some easy low light plants good for a newcomer to plants?
i really recomend amazon sword, they grow nice and big and will be perfect for filling lotsa space (i have one that's waaay too big for my 10 gal that i might be able to send you
) (OFL would be able to help you out more with types)
What is a good brand of filters?
i've seen a bunch of good reviews about aqueon and aquaclear, but i'd wait for someone else's opinion
Do I have to use co2 to keep my plant heathy and green?
What is a good brand of plant fertilzer?
a good substrate like eco-complete or seachem flourish will be a good starting point, but they tend to run $15-20 per bag, so for a big tank you'll really have to shell it out (eco-complete
on sale for $15, free shipping on $50 order at petco!!!)
And how many fish can be kept in a 55gal?
depends on the fish and size of your filter (http://aqadvisor.com/
good luck! post pics once you get it up!!!