”は” is a "Particle", a grammatical tool similar to a preposition. Particles are pretty much the only irregularly pronounced kana.
So, ”は” is the character for the sound "Ha"; but, when used as a Particle, it's pronounced "Wa".
Other such exceptions are ”へ”, which is pronounced "E" when used as a Particle and ”を”, which switches between "Wo" and "O" depending on accent and personal preference.
Look up basic particles if you want to learn more: they're quite easy to grasp with proper teaching.