What games are you playing? In my experience, there is a gigantic difference between playing single player and online multiplayer.
When I first played Unreal Tournament (1999) online, I was horrific and a total noob even though I'd logged hundreds of hours with the original Unreal single player game and even though I could play the single player UT game decently.
Unlike bots, human players don't run in straight lines and predictable routes and they'll try to outsmart you. They also have a great understanding of a map's choke points and open sniping areas, etc.
I think it must have taken me hundreds of hours merely to be able to just hold my own on the public servers and then a thousand more hours to become better than average and then perhaps more than that to become a good public server flag runner.
The first thing that I had to correct, and what helped me begin to make the jump, was to stop using arrow keys to look around and to start using my mouse. Then I bound "move forward" and "move backward" to the left and right mouse button keys, respectively. I also had to set my keyboard up so that I could easily use the strafe/dodge keys along with my fire buttons. (My setup is unusual, most people set the mouse buttons for primary fire and alternate fire and then use WASD for move forward, move backwards, and strafe left/right.)
Another thing--you can't run in straight lines and if there's a device available to you like the UT translocater in CTF, you have to become very proficient with it and learn to use it to travel across the map. (It's much easier than you think.)
Basically, there really isn't any substitute for hundreds of hours of online multiplayer experience. Heck, I'm still learning how to play the game better even today.
If you're playing CTF, I suggest starting with defense because you'll respawn closer to the action--closer to the area you want to play. Just try to make whatever little contribution you can. Perhaps you'll kill the opposition with suicide attacks first...then later maybe you'll kill the other players sometimes while surviving. Also, feel free to "flag camp". Hide behind a box or a corner and shoot at enemy players when they try to take the flag. Another thing you could try to do would be to get a spammy weapon and then go to a high-traffic or medium-traffic area of the map that has a narrow bottleneck. Camping the base's front doors might work.