AdaHopper Welcome! Hope this post helps 
How do I make friends?
It’s beneficial for you to have friends in your area that you play and raid with regularly, so you get bonuses when raiding and trading.
However, if you don’t raid or know anyone that plays, several players on this thread have put up their friend codes so you can add them. Do be careful though that when sending gifts it will mark the location of where you got the gift from and it is more detailed than it was previously, so if you’re worried about location be careful
What are x-raids?
Ex-raids were introduced last October. They are exclusive raids and at the moment only contain Mewtwo; it is the only way to obtain Mewtwo and likely always will be as it is the most powerful and sought after Pokémon in all the Pokemon games.
You need an invitation to an ex-raid, and the only way to get one of those is to meet Niantic’s parameters. Only certain gyms are ex-raid gyms, and it’s taken raid groups a long time to figure out which ones in their local area are ex-raid eligible gyms.
If you’re part of a local raid group, you’ll need to ask them which gyms are ex-raid gyms.
The main requirement is that you have to complete a raid at an ex-raid eligible gym in the week prior to passes being given out, however it is never quite clear when this is. If you have a gold level gym badge on the ex-raid gym, you are more likely to be invited to an ex-raid, as are those players who have completed a lot of raids in the previous week.
Best advice: raid a lot, especially at ex-raid gyms.
What else should I know?
Check out this guide for new and returning players: www.google.co.uk/amp/s/pokemongohub.net/post/guide/guide-for-returning-pokemon-go-players-2018/amp/
It is, however, two months old, so it doesn’t incoprate the new features of trading, friendship and gifts, so here is a guide on the newest features: pokemongohub.net/post/guide/friends-trading-and-gifts-in-pokemon-go/
(The most important thing to know about trading is that you can only trade with someone who you’re within 100m of, and the better friends you are, the cheaper the trade will be.)
Lucky Pokémon have only just been released in the last few days as well so here’s a guide on that too: www.usgamer.net/articles/30-07-2018-pokemon-go-lucky-pokemon-what-are-lucky-pokemon-how-to-get-them-and-stardust-costs
(Note: lucky Pokémon can only be obtained through trade.)