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