So, I need to add transistors for switching the Mosfets on with 12v? Anything else?
First off, I am not an expert on h-bridge design, whether mosfet or transistor-based, so take this with a grain of salt, listen to others who are, and do more research...
1) As 4pyros has noted, your high-side should be p-channels to switch the high-side voltage
2) Alternatively, you can do an all n-channel design, but you will need to use a high-side driver - either an IC designed for this purpose, or a charge-pump design
3) Note that charge pumps (and I believe some IC drivers?) generally need to always have a PWM signal; you can't just switch the mosfets "on" - the PWM keeps the pump "primed" so to speak; there are also limitations on speed of PWM, etc - based on the design and/or parts used.
4) If you aren't using logic-level mosfets for whatever reason, then yes, you can use a transistor to switch them instead (typically you would use the input voltage rail of the h-bridge; 10 volts or greater, whatever is being used to supply the motor)
5) Select mosfets of an amperage 3-4 times larger than what the motor is rated for (and/or parallel the mosfets if needed to get this level)
6) Remember to use heatsinks if needed
Once again - I am not an expert on this; this is all just from what I have researched on the internet in my own study of high-power h-bridges; in the end, I decided that if I ever needed anything over 10 or 15 amps, it was going to likely be better and cheaper to just purchase such an h-bridge...
Good luck - hope this helps!