Find your lens mount and your camera mount.
Identifying your lens mount
The easiest way to find and confirm your lens mount is to know the mount of the camera it was used on originally. A quick internet search of the camera model followed by lens mount is a sure way to confirm what lens mount you need. For example, if you have an old Nikon lens and you know it was used on a Nikon FM2, you can easily find that the lens has a Nikon F-mount. This is particularly useful for lenses that are made for different mounts i.e Sigma and Tamron bring out the same lens with different mounts including EF, F, E so knowing the camera the lens was used on is a good shortcut.
If you have an old second-hand lens and you are unsure of the camera it was made for, take all the known details of the lens and search the web.
Identifying your camera mount
A quick internet search of the camera model followed by lens mount is the quickest way to confirm your camera’s lens mount. E.g. If you have a Sony A7r, a quick search shows it has a Sony E-mount.