Need help with gears and wheel alignment
The rear spacing on most single speed frames is 120mm. The dropouts usually face backwards and can't take a derailleur very easily. Is your bike like this?
If so, a hub gear may be the best option. You can get a 2, 3, 5 or 8 speed hub gear that fits a 120mm rear end. This will maintain the clean look of a single speed, especially the 2-speed, that needs no extra cables.
If you try derailleur gears, you will need to create a hub setup that has enough space on one side for the sprockets/freewheel. You are unlikely to be able to do better than 5 gears, and mounting the derailleur will be a challenge unless you find an adaptor fit for purpose.