Hello, i started playing today. This is actually the 3rd time i install the game but the other 2 times i found it too confusing and quit

Now im having a blast, following advices on this thread i picked one of the irish counties, the one at the north east part with a yellow shield.
I got some questions though:
1)What are fosterlings actually for? Once they grow do i keep them in my court?
2) What improvements should i get? So far i got the forestry and fishing hut.
3) I got 2 friends, and 1 enemy, what are they actually for?
4) I had a claim on a title, which i took, but i got a notification saying my count only can control 1 demnse correctly. Should i grant the other county to one of my family members.
5) How does the classes loyalty work? (Peasants, knights, etc?)
Well that is all i think. Thanks.