How I should be able to kill enemies and level up (noob)

[Gyros]

As a beginner on a full normal server you should take over the support roles. Never lead a charge or something. Stay a bit behind your allies and only attack if it's safe to attack. If your team got Medic Boxes you can take those and heal your team mates by just dropping it ( Medic Boxes are considered throwables so just use LMB once). You can refill those boxes at the ammo box then. Healing is very important in later waves or Hard(lvl 32+)/Ragnarok(lvl39+) servers.

You can be useful in killing from the first class tier in my opinion. (Maybe archer not because they only get usefull from T3+ imo). However you should still be more of a reserve until you reach T3.

all in all just play safe and support your team.