To add to what Andy said, you have to apply some basic principles when mounting tires. The first and most important is that the rim diameter is least at the center, so you need to move the tire to the middle of the rim and push the slack ahead of you as you mount the tire.

Here are some hints.

1- inflate the tube so it's a sausage and stays up in the tire when you stuff it in. Later as you start mounting the second side, you'll need to deflate it so it doesn't push the tire outward on the rim.
2-startmounting the second side opposite the valve and ending there. This is possibly the real key to mounting tight tires because the valve keeps that section of the tire from moving to the center of the rim, thereby costing you a critical amount of slack.
3-when you finish, the tube near the valve may be trapped under the tire. Push the valve into the tire to pull the tube up with it, then PUSH (do not pull) the valve back down from the tire side.
4- inflate to 15psi or so to check that it's seated evenly. Spin the wheel and use the reference line molded into the tire as a guide. Once you've seated it evenly, inflate to full pressure.