If the tire has a tube the valve stem does not have to fit tightly into the hole. If you do not have tub in the tire and the stem is loose you will have to replace the valve stem. I recommend installing a tube into the tire. The tube can be ordered from this link.
. To install the tube one side of the ire has to come off the rim to get enough room to slip the tube into the tire. Once the tube is in, work the tire back onto the rim. Carefully do this so it does not pinch and puncture the tube.
You could also remove the tire and take it to a tire shop. They may be able to repair the tire or install the tube for you. Please resubmit your question if need further assistance.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services