the patriot- First of all, do you have an amplifier? If not, you’ll need to buy one, and they go for at least 50 dollars. Just something to think about.
Now, in terms of the actual guitar, for 100 dollars, your options are a bit limited. There are a few brands that make entry level models that are decent, such as Ibanez, Epiphone, and Fender. They can be anywhere between 100 and 200 dollars new, so you might find a used one for less than 100.
Another option would be to go to your local Guitar Center or Sam Ash and see what they have on clearance. Sometimes, they’ll have something marked down a ton, and you can save so much money that way. You may even find a 300 to 400 dollar guitar for around 100 bucks. Also, Guitar Center has their own brand guitar called Laguna, and they’re actually great quality, and are pretty inexpensive.
Bottom line, here’s some brands you should consider if you find them cheap: Ibanez, Epiphone, Fender, Laguna, Dean, Gibson (if possible), Schecter.
Good luck!