Have any tips on painting a room?

I just moved to a new place and I plan on painting our soon-to-be baby girl’s room. The previous renters must have had the opposite gender in there because the room is yellow and blue with sports decals throughout the middle.

So, we have a pink color picked out and I’m worried about how well it will be able to cover up the current colors. Will I need to apply more than two coats? Will I need a primer?

What are some other good tips for painting a room? What tools should I use? Mistakes to avoid?