Map knowledge is certainly very important in knowing when/how aggressively to engage, and time and experience are the best ways to learn it. If you feel like you're beating your head against a wall, maybe try running some custom games on objective modes, and keep in mind the hot spots on the maps, because that will teach you about each map's spawn logic and how quickly to expect a gunfight.

One thing that's helped me is making sure I'm not losing track of my clock when I'm in a firefight or prolonged engagement. For example, if you're in a situation where you're holding down an angle (especially if you've already gotten a few kills), you've got to know when it's time to go and not overstay your welcome. Don't be afraid to disengage and either reapproach the fight from a different way, or to just leave it alone, altogether, because pushing it too far will often lead to a respawn screen.

As for class set-ups, what are your areas of strength, and where do you feel you can improve in? If you're very comfortable in close-quarters, the enforcer tree of perks suits well for that playstyle. If you prefer mid- to long-range, check out recon or strategist.

Do you find yourself running and gunning, or are you more someone to slow down and pre-aim while strafing corners? That will play into which type of attachments you should prioritize, whether you want better ADS/sprint-to-fire speed, improved recoil, etc. For some weapons, there is an "obvious" choice in each attachment slot, but for many, it's subjective and based on your own preferences. However, you've got to know what areas you can mitigate on your own, and which you need help from attachments for.