So let me make this clear: Few decks are based purely on a type. They are instead based on a strategy. some strategies may only work with certain types, for example Bronzong decks used Metal Pokemon because his ability allowed him to quickly accelerate Metal energy. Also every deck contains Shaymin EX. All of them.
That said, it's very hard to build a deck with no knowledge of what I'm starting with.
First of all, you need to pick a format. This determines what cards are legal and what arent. Standard is cards from Primal Clash or later. Expanded is BW on, and Legacy allows cards between HGSS and Legendary Treasures, inclusive. I can't tell you what decks are going to be good anymore as no irl tournaments have been played on the new format, but I can guess.
If you are set on Grass types, I'd recommend a Yanmega deck. It will likely be one of the strongest decks in the format due to its speed and power. This deck uses Yanmega (SS 7)'s ability to deal 100+ damage for 0 energy as long as you have exactly 4 cards in hand, and gets set up quickly with Forest of Giant Plants allowing quick evolutions.
If you are just looking to casually play in tours though, you can run basically whatever you want. Just find a theme and stick to it! I've always been partial to Lightning decks, so that normally ends up being what I use. Also consider joining the Pokecommunity's official TCGO tournament for this month by signing up
here!