For DC motors such as fans, you can use a power transistor to switch power to the motor. An H bridge is probably over-kill since you only need the motor to spin in one direction. A circuit like this will probably suffice:
simulate this circuit – Schematic created using CircuitLab
When the GPIO is low, the transistor will be off and no power passed to the motor. However when the GPIO goes high, the transistor will conduct and the motor will run. The motor may produce "electrical noise" which may disturb the operation of the Pi by producing spurious signals. You can minimize or eliminate that with capacitors and chokes around the motor as well as using shield or twisted cables.
If the motor noise is still causing problems, an opto-coupler can be used to isolate the solution into two disconnected circuits.
I have also run into problems with electrical noise with the servos and again an opto-coupler solved my particular issues.
Should you need to control the speed of the motor, there are two solutions. First you can toggle the input to the transistor such that it is alternately on and off. If performed fast enough, then you will get an average voltage to the motor as ratio of the time on to the time off. Should you not need to control the speed of the motor, yet another solution would be to use a relay to control the power to the motor.