I would like to know whether 'I park my car 'on the porch' or 'in the porch' is correct?
In the UK, it's neither on nor in the porch. This is a decent-sized porch:
Parking on that is plainly ridiculous; and it's too small to park a car in it.
Andrew Lazarus suggested a carport, and it could look like a porch:
In British English, one parks under a carport; and if this structure were called a porch, it would still be under the porch. It's under because there is simply a supported roof, and no walls to be inside of.
What British English doesn't have is "porch" describing the sort of large covered terrace which fronts some American houses.