seems like a bit of a dead thread but I'll reply anyway....
The best thing to do is practice them separately and then bring them together when you can perform the parts well and sing with ease. If you can't do either in isolation then you really have no business in trying to do them at the same time.
When you do finally bring them together, a good idea is to actually simplify the guitar part. For example, instead of using a strumming pattern, try exclusively downstrums on the beat in order to get the timing right whilst singing.