KhanTF2Changing to a crosshair that's more visible might also help your aim a lot.
You can do this by going into "Options" and then clicking on the "Multiplayer"-Tab to the right. I'd recommend the cross, which is used by many people because of its simplicity and visibility. Green and Yellow are often chosen as colours since they are very bright and thus tend to differ from the background, meaning you see them better.
In general, for practicing your aim or other specific things, tr_walkway and tr_arena are training maps where you can train your aim against bots, which you can "program" to do specific things (like strafing, jumping etc.). To play these maps, just download them from the link, then extract them into your "maps"-folder, which is located in the "tf"-folder in your TF2-directory. After that, you can play them on a local server by either typing e.g. "map tr_walkway_rc2" into console or by clicking on the "Create" button and choosing the map from the dropdown list.
Yeaaaa i wanted to change my cross-hair. Thanks man and will def try out those maps to practice my shots.