If your only alternative is to trap, and remove an unwanted squirrel, you should first check with your local government agency. Your City, County, or State may require you to obtain a permit. There may be certain circumstances when a professional trapper is needed to capture and relocate the animal. A phone call can save you a lot of trouble.
The other very important part of trapping concerns the time of year. The female squirrel will give birth to a litter of three or four, in the early spring and possibly again in the fall. If you trap and relocate the mother squirrel, you will surely kill her babies since they can not leave the nest on their own. While this alone would be heart breaking, consider what will become of those baby squirrels. They can cause quite a smell. Please give this some careful thought before you proceed. If you're not sure, please ask for help. That's what we're here for.
If you are allowed to trap, your next step will be to find a suitable trap. Most large sporting goods stores will have a selection of live animal traps to choose from. Live traps sell for about $25, for a small "five by five by eighteen inch