I stink at this game, need help.

I have played all the total war games, and in every single version I have found that I am pathetic. I was hoping someone could give me a few pointers to make me a little better, I suck at both aspects, the real time battles and managing my empire in the turn based part. First off, should I be trying to uprgrade every city or castle I have, or should I just concentrate on building up a few cities and castles? How do I get my family members and generals to gain experience, traits etc...? I know the real time battles are so diverse that there are a lot of ways to win, but I don't seem to know any of them, is there a specific faq that would help me with the battles? I am sorry if this is too vague a topic, there are just so many aspects of the game and I just can't seem to figure it out. I do have two specific questions.

I love using assassins, but I can never seem to get any of them past about level 3 (by level I mean experience or whatever you call it). What is the best way to get their experience up so they are actually effective?

I am having huge problems with the papacy. I don't like catering to the pope when he orders me around, especially when he wants me to stop fighting someone. So I figured I would take an army to Rome and wipe out the papacy. I took the city no problem, and killed the pope. But then another one was elected, so I found him and killed him, and another took his place. Is there any way to wipe out the papacy or the papal states faction? If not what is the most effective way of getting my own priest elected to pope?

I am really sorry if this post is too big or vague, but if anyone can think of a tip or two that would help me get better I would really appreciate it.

Generals and family members gain traits in battle or after long periods of time at cities (sometimes good, sometimes bad). As for the real time battles, its just something you have to get the nack of, higher ground and good moral will all increase your chances of winning. IMO the battles in Med 2 seem to be the easiest to win out of the entire series, anyone else agree? I was a little disappointed.

As for the pope and papel states, I dont really understand how the system works yet. If I find out anything else I'll let you know.

There's some helpful tips on several of your questions in the forum FAQ:

https://forums.totalwar.org/vb/showthread.php?t=72109

In terms of winning battles, you might look at frogbeastegg's guide to RTW. Much of the tactical advice carries over.

Personally, I would advise specialising in upgrading one or two castles, so you can get the best troops soonest. Form a full stack army that is balanced - some archers, swords, spears and cav - and then follow "rock-paper-scissors" in battle. I tend to rely on softening up the AI with archers behind a shieldwall, then move in for the kill with swords and cav on the flanks.