How can you fix your toilet bowl if it leaks slowly?

If your noticing the leak coming from around the bottom of the toilet where it sits on the floor, then it could be that the wax seal maybe old and cracked, A plumber could help you fix that. If it's the actual toilet bowl that is leaking then you might have a crack in it and have to replace your toilet.

Depends on what you mean by "leaks slowly".

If it leaks water out the valve, causing the level in the tank to slowly go down, which then requires it to re-fill itself over and over again, you can replace the flapper valve, which is th rubber thingie that comes up when you push the flush lever. There are more reliable "tower" type of valves available that will last longer, leak less, and get hung up less often than the old fashioned flapper valve.

If you mean that water continuously flows, it's probably because the float is set too high. Water comes in, then runs out the overflow drain, which is the pipe in the middle of the tank. In that case, lower the float. How this is accomplished depends on the system you have. It might be by bending the rod that the float is on downward a little. It might be by squeezing some clips together so the float can slide down the rod that it rides on.