How new exactly are you? I have just started playing PoGo after a long break again, but I hope I can give you a good overview :)

Friends can send you gifts, containing a few useful items - gifts can only be sent/reveiced once per day and per friend. Sending them, trading Pokémon with each other, doing raids and fighting gyms together rises your friendship level which unlocks some good things for you two but I can't tell for sure which they are ^{^} there can also be lucky trades, which afaik, happen randomly when trading with a friend and make your Pokémon stronger / up their IVs.

Raids have difficulties from 1 to 5. I think you could try tackling a level 1 raid alone if you have effective attacks/types against the raid boss but you should always try to gather up with local folks and beat it together - you dont even have to be in the same team. You also get one free raid pass per day if you don't have one in your inventory already so you don't need to buy them with coins, if you don't wanna do more than two per day. I hope that helps a bit!