What Redrhodie said. Another tip: pump a little air into the tube before putting it on, just enough to inflate it a little. Put the tyre and the tube on like redrhodie pointed out and check if the valve sits right in its hole while you fully inflate the tyre (if it doesn't sit right you may have to deflate the tube again and wiggle it into place).

If you sit the tube inside the tire before putting the tyre on, there is a chance that you pinch the tube between tyre and rim.