It depends what you want to delete and what you want to keep. Is it apps? Is it photos? Music? If you don't want a lot of apps, hold down on one until they wiggle, then press the red x button on the top left corner of the app to delete it. If it's music, configure that in iTunes. Plug it into iTunes, go to your iPhone, and then music. If it's photos, go the your camera roll (not your folders below your camera roll, it won't delete the pictures, just remove them from the folder, but you won't gain any space) press select, tap the one you want to delete, and tap the trash can icon/delete button. If you have a lot of that so called "other" category, take a look at this tutorial about
wiping iPhone data. It provides 4 methods.