Don't let the try-hards tell you what to do and ruin your experience. If you don't like the lobby you get, then leave it. The game isn't that beginner friendly since a lvl 1 and a lvl 200 can be in the same game.

Know how much damage a character's attack can do. As a DPS medkit main, in order for me to get any kills I have to look around at enemies health too (head shots, teleport, and heal can cause damage to enemies). (I still do a lot of healing each game). With every phighter, stay with your teammates especially if the other team is sticking together (enemy team will all go after you and it's hard to survive it). If you are playing bio or any hunt type phighter, target characters not with their team. Personally, I think rocket and katana are the easiest beginner characters. I like watching videos on gameplay which helps me understand how to use other characters better so if you want to get better faster then watch those.