Anyone can be a partner. You do not need to join a network to make money you can just use AdSense. There used to be requirements to be a partner. But now the only requirements are no copyrighted material, follow community guidelines, and you can't live in Turkey and other countries. Most the country's where you can't be a partner YouTube is banned there anyways. To be a partner, just go under your account settings and click enable monetization, doing this and verifying a phone number should enable custom thumbnails as well. Once you create and link an ad-sense account to your channel you can get paid, but you won't get anything until you get $100 (for US, different depending on the country) which would be at the very least 20,000 monetizable views, but probably much higher because when you are starting you get a low cpm anyways. All the ads go to popular videos so don't expect much. Yes, you need to enable monetization to be partner. You can still be partner if you don't link with google adsense but you won't get paid unless you do, so of course you shouldn't link an adsense account. Keep in mind you do need to be over 18 to have an adsense account, if you are under you need your parent permission.
Networks are a whole different thing. They take a percent of your money to give you support and extra features. Some have lower minimum payouts then $100 and some have ad sales teams that give you more ads and supposedly a higher cpm. Most have requirements you have to meet to join. If you are going to join a network be sure to read the contract as well as reviews for the network so you know what you are getting into, as once you join you usually aren't allowed to leave until your contract is up. If the network does not have an ad sales team they will not get you a higher cpm, it is the same money you would get from adsense, actually less because they take a cut.
