i don't know anything

so my friend was telling me to try this D&D thing so here i am but i don't know if i need to download anything play, so i was wondering if anyone could tell me what to do 

You don’t necessarily have to download anything! There are tons of resources all over the internet that’ll teach you how to play. I learned a lot from the youtube channel called Don’t Stop Thinking. Please give it a shot, there’s loads of beginner guides there!

To play you need a group of at least 2, but optimally more. The more standard size is about 4 players plus a DM.

A game will consist of a DM or "Dungeon Master" and the players. Each player will have a character they control. The DM will be: all other characters, the one who "runs" or manages the game, who develops the story, the world and resolves the things your characters do.

There is a bunch of things you need to get started and play in the Free Basic Rules which you can find here: https://www.dndbeyond.com/compendium/rules/basic-rules

thank you both

No problem! Happy gaming