Most cases will come with a few different packets of screws and they may have different sizes or threads, so make sure to match them up with the correct mounting locations as best as possible.
Keep any screws which were supplied with the case separate and take note of the different types. Most cases will come with a few different packets of screws and they may have different sizes or threads, so make sure to match them up with the correct mounting locations as best as possible. If in doubt, refer to the documentation which came with your computer case.