When are you the happiest? Is it when you are photographing people? It may make you happy to bring out the joy in others. If so, photographing people is probably the best idea for you.
Do you enjoy taking in the views of the sunset, the waves rolling onto the sand, leaves falling from the trees, or piles of snow piled up on the side of the road? If nature makes you smile, you may want to consider making nature photography your specialty.
If you are a cook and/or a baker and enjoy looking at the crisp photos of food, then specializing in food photography may be the way to go.
Whatever it is that brings a smile to your face and warmth to your heart, is just the thing you need to photograph.
Now, start taking pictures of everything you like. It is never too soon to start building your portfolio.