how to increase space

Yours is not an unusual problem. The only way to retrieve a significant amount of hard drive storage is to clear off (delete) sufficient files which are currently stored on the HD. My suggestion is to do the following. Purchase an external drive so you can copy files (such as music, photos, etc.) onto it. Then those files can be erased from your HD to clear the needed space. These days good external disks are readily available, compact, cost effective and easily installed with a vendor provided USB cable. Most do not require any external power source. You may have to reformat it first, though. Easy solution. Then you will have plenty of space for your future needs. You should also know that if a HD gets too full it can stop functioning.

have you read

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support (IS)