Aliteration is when the first letter (s) of a few words are the same. Onomatopoeia is the sounds something makes.
Eg for both-:
Aliteration...
Jumping Jack jumps jollily!
Onomatopoeia...
Splish! Splash! The girl jumped in all of the puddles!
I think it is kinda obvious what the difference is now!
Hope you understand now!
Jasmine xxx