how can i use a fet as a switch.
what are the parameters that should be considered before designing.

 

If you execute a search for "fet as a switch" on this AAC forum, you will get 219 hits. Start there.

 

Are you talking about a MOSFET as a switch, or a JFET?

JFET's aren't good switches, while MOSFETs are. There are many threads on MOSFET switching.