Hi @Hope Komla. Welcome to the forum!!!
When it come to staple guns Im a die hard BeA fan. I use the BeA 71/16-421 and love it.
https://amzn.to/2LfsJxB
@BigRig has a different brand he uses and likes. Its a little less expensive. What one do you have BigRig??
Most upholstery staple guns take 3/8inch crown staples. The lengths come in 1/8 to 1/2 inch. Length depends on your project.
Use stainless staples for anything outdoors and regular staples for anything in doors.
I recommend getting a small air regulator that you can install on the gun so that you can quickly adjust the pressure. Air pressure sets the depth of the staple and will change depending on what your putting staples in to. You want the face of the staple to sit flush with the fabric. If its deeper than that it might be cutting the fabric. If it sticks up higher its only holding the fabric by the 2 small legs of the staples.
Hope this helps!