No idea where to start

One of the most important things if your new at this is to make sure you drill out the spot welds on the old floor pan as well as the ones on a donor car if you go that route. If your floor pan has alot of rust you can take some sandpaper, file or whatever you can find to clean the area where the welds are so there easy to find. And another note of interest these spot welds are not uniform, by that I mean they are not welded every 1- to 2", there is a pattern but they just dont fall perfectly in line.. (simple I know, but I found it out the hard way.. ) Highly suggest you go spend the $12 - $15.00 for the bit thats designed to drill out spot welds. It's worth its weight in gold.. It will only leave a 1/8" hole or so to close up after you drill the spot welds out.. And yes you will need to close them up with a welder and grind them flush for the new pan to fit and not rust through again from the drilled holes.
I went thru AMD metals for my pan, had it shipped to JEGS in Columbus Ohio, drove up and picked it up to save the $130.00 shipping charge.. I wasn't fortunate enough at the time to find a good floor pan that suited me, without it being shipped across the country..
Have fun, take your time, and ask all the questions you can think of.. We all do! LOL!!!