Welcome to the beautiful and complex world of planted aquariums. I sincerely mean it and I don't want to discourage you. Look for "aquascaping" or "planted tanks" in youtube and you'll see some beautiful stuff. I've been trying to do it for the past 2 years, and it's been an interesting experiment. sometimes rewarding and joyful and sometimes it's been a heartbreak.
Plants in general need a substrate which their roots can hold on and stick to. If your substrate consists of smooth pellet sized rocks, then the chance of success is small. There are bunch of vidoes on youtube regarding the substrate that you need for a planted tanks. There is eco-complete, ADA Amazonia, Seachem Flourite, and Fluval Stratum. They all have pros and cons attached to them..(financially or benefits to the plans). I suggest you to youtube it or just do a search on aquriumadvice.com. There are also plants that don't need substrate at all. here is a list:
Hornwort: very nice looking and easy to take care of.
Java moss: you can glue or tie it to a piece of rock or wood.
Anubias: same as Java moss.
good luck