can I add more space on my start up disk? like purchase it

No, sorry. With the MacBook Air, you need to purchase what you need in storage when you order the device. Its not upgradeable afterwards.

Look for storage you can safely delete under your home directory. Thinks like photos, music, video can consume a lot of disk. Pruning your photo library of duplicates and bad (pictures; such as badly blurred, shots of your foot, when you clicked the shutter at the wrong time, etc..; Home videos that are not of interest to you and your family, etc...)