Watch the Changing Minor to Major online guitar lesson by Matthieu Brandt from Songwriting on Guitar
There are many ways to add to the six chords you have at your disposal in a major key (three major and three minor). If you don't want to stray too far from your original key, but like to give certain sections some extra forward momentum, you can alter the minor chords from the key to major chords.
The V-I relationship that exists between two chords from a key is the reason why this works. This is the strongest harmonic movement we know. Aside from the dominant V chord going to the tonic I chord there are three other locations within a major key, where this relationship exists. In these cases all the jump-off chords are minor. You can strengthen this urge by making this chord major. This - as it's called - secondary dominant will have some tension in the original key, because it contains a note that is not part of the original scale; the minor third will have been altered to a major third.
The chosen melody will have to take this alteration into account.