A lot depends on the level of your study, and the context. If you are at secondary school and not studying computers as a specialist, then the answer may be simply to read the labels on the back. Power consumption and fuse ratings are probably given.
Three things are important: Don't use a fuse lower than the current drawn by the device, don't use a fuse higher than the level that would prevent nuisance blowing, don't use silver paper or other means of circumventing the fuse.

Stu