Welcome to the board!
It usually shouldn't be a struggle to get the belt on - if it's one of the old black cast-iron machines, you can loosen the motor mount and slide the motor up to get yourself some slack. I have a White that you have to loosen the flywheel instead, but *something* should come loose to allow you to get the belt on and off pretty easily.
The belt should be much looser than an automobile belt, for example. You want it as loose as it can be and still catch a grip. Like Joe said, it should LOOK loose; if your belt is making perfectly straight lines to and from the pulley, it's too snug.
If you can post a picture of your machine, someone will probably be able to give you very precise tips on what to do.
