What's the best way to clean/prep a dirty, moldy, peeling porch.

Borrow or rent a power wash machine (remove all contents of porch). Hopefully this is not an enclosed porch and doesn't have any carpeting. Otherwise you may also need a shop vac to suck all the paint chips away and any water. To it on a hot day, open windows and doors to air it out and use fans. BTW: Presume there is LEAD in the old paint--use face masks, no kids or pets around and make sure you get EVERY bit of the removed paint out of their reach.

Oil base sealer first.

you will never get all the old paint out of the cracks. Do your best with a scraper and orbital sander to get loose paint off. Thoroughly power wash and be sure wood is fully dry before painting. Make sure you are using a primer compatible with your paint. Once a deck has been painted it will be a perpetual maintenance issue. Every 2-3 yrs. You can prolong this a bit by not painting too dark, as that absorbs heat.

Use a putty knife to loosen the debris in the cracks, then a power washer to clean the deck. If you don't have one, perhaps you could borrow one from a neighbor or rent one for a few hours. It is so worth it to do it right or you will be doing it again in a couple of years. I have been told that the prep work is the most essential part to a proper and long lasting paint job.

Power wash, scrub with straight vinegar then power wash again, then see what still needs to be done.