To increase any skill, you just have to get essence from the Smurfettes (that is, you have to engage in frisky situations with them). In the particular case of strength, you can get it from Sassette and Storm; at the beginning of the game, the easiest way is by playing with Sassette in the slider, once you have some heart points with her there's an option that will give you strength essence. In addition, when you visit Blossom in the afternoon, you will gain points in a random skill.
Thank you for playing the game!