Assuming you're talking about a sink faucet:
If your faucet handle feels loose or wobbly, there's a good chance the set screw is loose. The set screw is located under the cap on the top of the handle. Use an Allen wrench to tighten the set screw. If this doesn't solve the problem, then the issue is probably with the valve stem. To fix this, you'll need to remove the handle. There should be a retaining nut that you can unscrew with a wrench. Once you've removed the retaining nut and handle, you should be able to see the valve stem. Tighten or replace the packing nut and washer on the valve stem. This should solve your problem!