how can i extend my 8gb memory space on iphone 5c
You can't expand the memory, and there is little in the way of dealing with external USB storage other than importing photos from camera cards. 

There are several products such as the mophie space pack that have a battery and external storage that is accessible via an app. These are typically cumbersome and fairly limited in how they can be used. Check out reviews of any such products and you will see lots of disappointment. 

For what these external storage devices do, you are typically better off using iCloud, Dropbox, getsync.com or the like. But this will not solve the problem of not have enough local storage.