Does anyone have a quick & easy way to clean wooden porch for paint?

I would use a power washer... let it dry, sand an rough patches and then power wash again. Let it dry thoroughly before you paint!

You could rent or borrow a pressure washer.

The easiest way would be to power wash it. If you don't have a power washer you can mix a cup of vinegar into each gallon of water to fill your cleaning bucket. Dip a broom or scrub brush into the solution & wash in the direction of the wood grain. You will likely need a sponge, rag or old toothbrush to get into the smaller areas. Allow it to dry completely before painting.

TSP was actually designed as a deck wash. Use with pressure washer if it's really dirty.

Buy paint with primer in it already