I recommend Tony Luke Jr.’s. It’s a spin off of the original Tony Luke’s in South Philly, which is superior to both Pat’s and Geno’s. Jr.’s is on 18th Street somewhere between Chestnut and Walnut. It’s a short cab ride from 30th Street Station where Amtrak lets you off. If you’re taking NJT, get off at Suburban Station. It’s only about three blocks south of there.
If you really want the Pat’s and Geno’s experience though, and there is nothing wrong with that (it’s fun and it’s not like the steaks are horrible…they’re good; just not among the very best), then you’ve got a longer cab ride from 30th Street Station. If you take the El (the east-west subway essentially) west to 8th Street, then head south, you’d have about a 15 block walk to get to Geno’s and Pat’s. It’s also about that far from Suburban Station if you’re taking NJT.
As far as ordering, you say two words. The first is the kind of cheese you want (Whiz, which I recommend, Provolone or American) and the second is "with" or "without," meaning with or without fried onions. I generally get whiz with.