In English we say "I listen to my mother". In French they say "J'écoute ma mère". A French person might ask, "Why is there a 'to' between 'I listen' and 'my mother'?"
For a more detailed explantion, see here:
Prepositions are the bane of translating.
ThanKwee is completely correct, but the simpler answer is that ressembler always takes an "à"^. So you can think of it as "ressemble à" translates to "(he/she/it) looks like" and you never^ have ressembler without the "à". You can think of it as part of the word (although you can also have words that go in between like "bien").
^(unless it's saying "these things look the same" in which case it's used as se ressembler)
"Ressembler à"... these first words have to stay together to... next "a boy" un garçon