A lot of that is variable based. How big of a house this is determines the base costs involved. Location comes into play as well as well as who you're asking to help you paint it.

As for the main steps
Step 1: Clean the walls with a washing to remove dirt and cobwebs
Step 2: Repair damage with chalk to seal cracks
Step 3: Protect doors, windows, and lights.
Step 4: Apply primer
Step 5: Paint with roller or spray

should the old paint be removed first or should i just overlay them with new one?

It depends on how bad the old paint is. If it's really peeling at some obvious parts, then you can go with the removal, but if it's just in small areas, you're better off just applying the primer. The primer is used to seal in those areas with the cracks left by the old paint and provide you a "flat" surface to apply the new paint on.