I have a question regarding the relays, why do some of them cost something? 
Discussion
By using paid relays you decrease the possible amounts of spam. Most spammers don’t want to pay (even a little) to spam. So by using these paid relays the signal gets better and you see less noise
And how do I choose on which relay my „tweet“ shows up
Tweets are called “notes” here.
They show up on the relays you are connected to. If people ‘boost’ or ‘repost’ your note it also ends up on the relays those people (and other people) are connected to.
Ahhh okay, and do you know how to search for people? I can only search for hashtags
You can find people by checking who other people follow and start there by following people you think might be interesting, or you can use https://nostr.directory to search people based on their Twitter handles