The major tool suppliers will kit up a "homebuilder tool kit" that has enough stuff to get you going. Unless you have a garage full of stuff already, it's probably worth looking at one, either to buy in one click, or just to get an idea of what's considered essential
Off the top of my head, here's some stuff you're going to need right away;
A good rivet gun- this isn't a place to skimp Buy a good one that you can feather the trigger on. Note I said "good," not necessarily "new & expensive" my favorite rivet gun is a 3X that I got used from the Yard Store in Wichita. They always have a selection of refurbished guns that originally lived in the Cessna or Boeing factories. Some of them look beat to smithereens on the outside, but they've had the guts changed out they shoot like a dream
A selection of bucking bars- I LOVE the little tungsten bucking bar. It's crazy expensive compared to steel, but it's crazy easy to use compared to steel as well.
a decent air drill
#30 & #40 drill bits
A compressor- the higher volume, the less time you're waiting for it to pump up. I have a Cobalt 26 gal. vertical from Lowes, and it will keep up wit pretty much anything but a die grinder.
A way to measure stuff down to 1/32" Also not place to skimp. I have a 4" steel ruler graduated in 1/8" and 1/16" on one side and 1/32" and 1/36" on the other side.
Dimple dies and a way to dimple (i.e. either a c frame dimpler, a DRDT-2, or a home brew solution.
About a zillion silver clecos
1/2 zillion bronze clecos.
A good vixen file, various grit sandpaper, red scotcbrite.
Stuff that you're going need eventually:
A 4-5" bench vice
rubber hammers
lots of different kinds of clamps
a bench grinder
torque wrench
Stuff that you can live without, but it's really nice to have
Drill press
bench top belt/disc sander
bench top band saw
bench grinder/buffer with 3m wheels for aluminum
45* & 90* air drills
This list inset all inclusive by any means, but hopefully it will get you pointed in the right direction.
Also, nobody always has what they need when they need it. When that happened, you can take a step back and buy borrow or build the right tool, or you can take a step back and figure out how to fix what you just botched up with the wrong tool.