How to know how gb my iphone 4 has in total?

It says 6.3 gb capacity and 263 mb available what does that mean?

Go to settings>General>About

Settings--> General--> Usage

It will show you what you have available and how much is used.

That is what space you have available to store things on your phone.

and if the numbers get really low eventually you just need to delete unwanted stuff.
things on your phone are using the gb and mb's.

Its an 8GB model

There are 1024 bytes in a kilobyte, 1024 kilobytes in a megabyte, and 1024 megabytes in a gigabyte. Just think of 1000 Megabytes = 1 Gigabyte, it's easier that way.