Raising versus flatting when should it be done on each street?

I struggle with knowing when to raise. I tend to flat and wait to see what shows up on the board, or let others bet and just call. How can I force myself to be the raiser, and when I do that? Should I only raise in late positions? Are there specific hands that I should have to raise in early position? What am I hoping to accomplish with raises, build the pot, cut the number of players and so on. So far, my limited thinking is to raise early if I have middle pairs or vulnerable cards, and flat or raise late with my stronger hands. What should I be doing?

Hey, I'll give you a tip, here in the forum has a very good course: Tournament a Winning Poker Player in 30 Days

Link: https://www.cardschat.com/become-a-winning-poker-player/

The course is excellent, with simple and objective language, as well as ebook material and video classrooms. I'm fazendo and I'm enjoying it, I recommend.