Where to start?
Well, I'd start with the pipe-check. If the underground pipes are leaking you should fix them first, because nothing else is reasonably possible until then.
Then comes the condition of the liner. If it's intact and not leaking you are in good shape. If it's torn you'll probably need it replaced, so do that before adding water.
After that comes the filter and pump: Are they OK and running and ready for the season.
When you are good to go on these three, then you can start the process of cleaning the pool.

You'll need enough water to run the pump--over all the returns and drains, including the skimmer.
You'll need to scoop out debris as much as possible. Use a pool tool called a "leaf rake" that's like a cross between a squeegee and a butterfly net.
And start that filter going.

You'll need to test the water to get a baseline. The best investment you'll make (besides your time on PoolForum) will be in a FAS-DPD test kit. You can get a Taylor K2006 (not the 2005) or the Leslie FAS-DPD Chlorine Service Test Kit. Ther'es a way to get the Taylor kit from Amazon(?) that gives a commission to the owner of this site (Not me, Watermom, Poconos, or aylad). For $60 or $70 bucks it will save you thousands over the years, plus innumerable headaches.

And you're gonna need a LOT of bleach, or Liquid Chlorine (same stuff as bleach only more concentrated).

That's enough for now.