Hi Zakkary123 and welcome!

Ash is a great looking timber and great for guitars. It is a very open pore grain though, and needs to grain-filled if you want to go for a smooth mirror-like finish. You'll be able to finds heaps of info this process here and around the interwebs. There are a few ways to accomplish this and after a bit of research, you can decide what is the best for you.
FWIW, you do can skip grain-filling and get a more rustic look. (It's a thing, and there's no right or wrong there!)

As for sanding between coats of Tru Oil, everyone has their own sort of process of applying it, but for me, I don't touch it with any abrasive until it has started to develop an even gloss, which can be 4-6-8 coats depending on the type of wood and how the TO is applied.
And when I do start to use an abrasive, it is a very light going over and a fine grit. Just enough to smooth out any dust nibs. I prefer synthetic pads to sandpaper or steel wool. (personally, I avoid steel wool like the plague)

The key is multiple thin coats, not a few heavy coats. In the right conditions (temperature & humidity) I can get 3-4 coats on in a day.

Best of luck. Look forward to seeing your progress. Lots of helpful folks here to get you through it!

Cheers,
Mick

edited to add: Good advice from Sonic there!