How do I add text box to photo??

Currently you can't wiithin Photos. You need to export the photo to the Desktop and edit it with a 3rd party image editor or use Preview's annotation feature. Once the text is added to the file save it to the desktop and import into Photos as a new file.

Some Image Editors That Support Text and Layers:

Photoshop Elements 11 for Mac - $79

GraphicConverter - $40Rainbow Painter - $30

Affinity Photos- $50

FX Photo Studio - $30

Inpaint - $20
Imagerie - $38
Acorn - $50

Pixelmator - $60

Seashore - Free

GIMP for Mac - Free

If you don't want layers, you can use Preview: it's not only free, it's already installed with OS X. Export your photo from Photos (stupid name), then open it in Preview and click on the little toolbox icon in the toolbar to open up a set of interesting tools with which, among other things, you can add text wherever you wish in your choice of colour, size, and letter-type.