Could you be a little more specific on where you are having trouble getting the tire onto the rim?
Also possibly what tire you are using?
Getting one side of the tire should be fairly straightforward, but the second side might be a bit trickier, especially at the very last couple inches of the tire.
I have almost never had trouble with tires if I do it in a certain set of steps:
1. Lay the rim/wheel as flat on the floor as possible
2. Put the inner tube lightly pumped into the tire, then lay the tire+tube combination on top of the rim.
3. Place the valve stem into the valve hole in the rim. Work ONE side of the tire onto the rim. It should hopefully mount without any hassle, but if there is any hassle, a simple tire lever can get it over.
4. Now that one side is on the rim, orient the wheel so it is vertical, like it would be when you are riding the bike. Make sure that the valve stem is on the top and begin using your PALMS, not your thumbs, to mount the tire onto the rim, starting from the valve stem.
Basically you should be standing over the rim, and work your way downwards.
When you get to the last few inches of the rim, hopefully you can get it on with no tools, but if you need a bit of extra help, use a tire lever(s) to mount the final part.
If you get really good, you can mount/dismount tires without any tools! It's really impressive for others to watch, if you're into that kind of thing