First of all, all starting weapons are actually among the best weapons in the game (m16/ak74m, aks74u/m4a1, m27iar/rpk, svd/mk10). What you want is to unlock attachments to these weapons. You'll want a Kobra or Red dot sight to acquire targets more easily. Then you'll want a heavy barrel equipped 9 times out 10. In CQB-maps, you may want a grip as well. This goes for all weapons except sniper rifles (you can keep them pretty much vanilla).
The key to kicking ass is to 1. use your weapon effectively (fire in short bursts, learn how the weapon handles, reload at the right moments, use the right attachments etc) and 2. to traverse the battlefield in a smart way which means moving cover-to-cover, avoiding open areas, iron-sighting pre-emptively, trying to meet the enemy where they are at a disadvantage and always anticipating from where the enemy is likely to come at you.
That's a few pointers anyway.
Keep in mind that you are pretty late to the party, so that's going to be a handicap. GLHF!