New to ck3 and need some tips?

Hi guys so I just started playing like two three days ago, haven't played the previous ones, and im entirely new to it. I had a couple of questions and if anyone could help me id really appreciate it.

1. Is there any way to increase the military considerably? Or how can I understand it better? I see the number is around 3k or so but then I see the soldier number is different? I just want to know cause I keep trying to make my English kingdom accurate but the french empire keeps bothering me with their massive military.
2. I think it's related? But should I be paying attention to the buildings in the county's and such?
3. Is there any way to make vassals less powerful or assure that no one will try to claim the throne on succession?
4. Is there any way to be able to murder or arrest more easily? It's always on very hard, I'm guessing this has to do with the trait related to this but I'm just wondering if there's another way?