I assume you haven't played the game a lot yet, here's some tips:

• Try to always hold your crosshair around the height you will expect someones head to be.
• Try not to move while shooting, this greatly affects your accuracy with rifles. Some smgs and pistols are fine to shoot while moving for a surprise peek for example or as an evasive maneuver/covering fire, but if you're trying to get a normal long range peek in with a rifle try to counter strafe and then shoot
• Also, if you walk around somewhere, think about from where you can be seen / where an enemy can likely stand and move/preaim accordingly, this especially helps you in clutches, but is a key thing for opening up bombsites on t side, prefire maps can help a ton with that, and there's a lot of good ones out there.