Depends on where it is sir.
If it's on the exterior body work I'd personally slap a bit of rust treatment on it (Hammerite Kurust for example) let it do it's thing and dry, then touch it up with paint just to try and seal it, then get yourself over to a bodyshop to get the job done properly before it spreads any further, as quickly as your wallet allows it.
If it's anywhere inside the car, Boot, under carpet, behind bumpers etc, I would sand the rusted area back to bare metal, rust treat said area after for added protection and then use a multi-stage kit such as POR15 in the colour of your choice to help prevent it coming back again.
Quite a few members have done this now with decent results, For a better Idea take a look half way down page 2 of my build thread (Link in my signature below) of when I did my boot.
Hope that helps.