I'll get you started with a few examples.

We have $F_3 = 2$. Sums to $3-1=2$ using $1$'s and $2$'s are: $$1+1, 2.$$

We have $F_4 = 3$. Sums to $4-1=3$ using $1$'s and $2$'s are: $$1+1+1, 1+2, 2+1.$$

We have $F_5 = 5$. Sums to $5-1=4$ using $1$'s and $2$'s are: $$1+1+1+1, 1+1+2, 1+2+1, 2+1+1, 2+2.$$

From here, you should then try writing out some additional examples to see if you can understand the pattern and how the next set of sums is related to the previous two.

Let us know if you have additional questions.