What condition is the siding in now? Do you have a lot of checking or splitting? Paint flaking or chipping?
There is line of primers for this type of application. They are very thick and almost look like elmer's white glue. Zinsser Peel Stop and Sherwin Williams Prime Rx are two that I have used and both work well.
If you don't have these issues, then just an acrylic exterior primer. Zinsser Bulls Eye 123 is excellent, or anything from SW. My HD sells 123 in 2 gallon pails for $30. That's a bargain.
If the siding is not painted yet, scuff sand with some 40-50 grit sandpaper. This will help with checking down the road. This is a recommendation from the US Forest Products Laboratory.
I hope this helps.