It looks like you are trying to build an H-Bridge.See this
Code:
[I]**broken link removed**[/I]
You need to pull an N-Fet gate above the source by at least the data sheet threshold voltage (Vths) usually with a safety margin as well, and you also have to be sure that at the Vgs you are supplying that the Mosfet will be able to pass the amount of current you need (this can be found in the data sheet under the Vgs,Ids curves). Then you have to make sure your drive circuit can supply the current you need to charge the capacitance at the gate to switch your mosfet on fast enough (source current) as well as sink current discharge the capacitance quickly enough for your switching speed. What you need is a high side driver either an IC or using discrete components. If practical you could use a P-FET Vgs pulled below the source. There are more considerations but you can read about them on the links provided.