How to use an external flash on the nikon d3000?

I've got an external flash and i've attached it i just don't know what to do next like how to get it working and all that.anybody know how?

Added (1). It doesn't have instruction i got it from a friend but don't worry about it.

External flash units usually come with instructions… Try reading them.

It would help if you said what flash you have. If it is a Nikon flash you should just turn it on and it will work fine. The flash will just replace your pop up flash. If it is another brand of flash it will depend on whether it is TTL compatible or completely manual. You would need to read the manual that came with the flash.