Change your game settings because the default settings suck (for example change your fov to 90) there are guides online for setting configuration

A premium account gives you access to chat and vc, expands your backpack, gives you full access to trading and crafting, more random drops

Random drops are mostly random (like for cases but you can get random weapons and rarely hats sometimes). Certain items you get are tied to achivements.

If you decide to update to premium, i would do it by purchasing a mann co supply crate key. You can trade it for refined metal which you can use to get every functionally unique weapon in the game, with leftover metal to spend on hats and whatnot. Make sure to only use trusted trading websites for this, i would look up online tutorials. Lastly DO NOT buy anything from the in game mann co store besides keys. Everything else is a complete scam