Please give more info.
What type of knife?
What is it used for?
What sort of edge do you want?
What is your level of skill with sharpening?
What shape is it in now?
Youtube is a great place to start. There are many sharpening systems for different types of knives.
My favorite are the two paper wheels that go on a bench grinder. Great results but misused they can destroy a knife in short order.
It's smaller on the outside.