Where to start....well you need to login to vSphere. Maybe the code below will get you started...it's in c#.
VimClient vSphere = new VimClientImpl();
vSphere.Connect("https://vSpheremachinename/sdk");
vSphere.Login(username, password);
Once you are logged in, you can query a vm with either findentityviews or getview if you know the MOREF.
After you have a vm object you can use: VirtualMachine.PowerONVM_Task();
Hope this helps,
Josh