This isn't actually math, so I don't feel to bad about giving a big hint.

1

Describing the above, we have "one 1".

11

Describing the above, we have "two 1's".

21

Describing the above, we have "one 2 and one 1".

1211

Describing the above, we have "one 1, one 2, and two 1's".

111221

Can you see where to go from there?