To increase your strength, you'll have to collect many Pieces of Heart throughout your adventure. This is a list of the locations of all 12 Pieces of Heart.
1. (1-K)
2. (2-I)
3. (5-E)
4. (3-G)
5. (15-M)
6. (9-H)
7. (15-C)
8. (14-M)
9. (7-H)
10. (8-A)
11. (2-A)
12. (7-I)