What frequency?
What voltage?
What power?
What prime mover are you going to use to drive the generator?
What resources do you have available to build this generator?
And a few other things as well....
The above link shows how to make one using a lawnmower, automobile alternator, and a 12v battery w/ inverter circuitry. After you fart around gathering the parts, doing the assembly process, you could buy the neat portable genset at the bottom of that website's page for a mere $169. Then you'll have 1kw of AC power as well as 12volts dc at 10 amps for other uses. And the whole thing can be transported much easier than a lawnmower engine operated contraption! Think about it.