When I first started, I was told that you should first build up 5 (?) 3* teams. Then do the same with 4* and then 5*. Until you have enough 3*, you can also use 2* and even level them up, but replace them with 3* as soon as possible. After that, 2* are fodder, as 1* always are. But there are always exceptions.
Be economical with gems. Use them selectively when you need them. Your roster space is limited and expansions cost... gems. And you get a discount on 10 pulls.
Healers? They are definitely helpful at the beginning. But I mostly only use them for defence now. Find your own way.
You can't focus on just one thing. It's all connected.