Reddit mentions of D'Addario EJ73 Phosphor Bronze Mandolin Strings, Light, 10-38

Okay, so...

Watching the clip /u/TheJiggersUp posted below, I don't see him playing his mandolin, so I don't have much to offer on style... I would presume he's going to play in a more classical/European style, as opposed to an American, bluegrass-oriented style. He has a bowlback mandolin rather than a flat-back or carved-top style instrument, but I don't know of any playable bowlback models that cost ~$100.

What does exist is the Rogue RM-100A which is a not-great but more-or-less playable mandolin for under $100. (Some individual instruments seem to be better than others, but it's just a luck-of-the-draw thing.) If you were to get one, you'd also do well to get some different strings for it, get a decent pick designed for mandolin (a guitar pick won't work well for this instrument), and maybe consider having it professionally set up (or possibly even trying to do the work yourself... perhaps someone else can point me to the book available on setting up a Rogue to be less awful.) I would also highly recommend getting a chromatic tuner; you spend quite a bit of time tuning this instrument, and as a beginner, it's really helpful to have one of these. If you have a smartphone, a metronome app would also be a good thing to get; otherwise, there are a million cheap, electronic metronomes on the market.

So the Rogue is a cheap, low-quality instrument -- NOT a good instrument, just the least-bad at the sub-$100 price range -- but it can get you started. I know, because that's how I started. It's designed for bluegrass more than traditional mandolin music, but you can learn the rudiments of any/every style on it.

After that, to just start learning how to play, I recommend Don Julin's book. Yes, it's "for Dummies." It's still an excellent beginner's book. He focuses primarily on American styles, but he also covers other mandolin traditions, and when you're first starting out you really just need to learn the basics no matter what style you favor. He also does some nice intro-level youtube lesson videos. There are lots and lots of other mandolin lessons available on youtube, too, and are a decent way to start out if you can't afford private lessons.

Finally, start saving up some money for a better instrument. Sooner than you can imagine, you will reach the point where the Rogue can no longer keep up with you, and begins to hinder your progress, so you need the upgrade to keep learning. The most common recommendations I see are the Eastman 304 or 305 and the Kentucky KM-140. Both are solid student instruments that will serve you well for a long time, and both can be had with professional set-up and a hard case for ~$450. I have an Eastman 305 myself and I adore it. Or perhaps there's a more traditional bowlback model out there that you'd like better, if you aim to play that way. I know very little about those.

It's a great instrument, and fun to learn. And it's entirely possible to come from zero background with stringed instruments and still make good progress. Good luck!

Edit: Oh, also... certainly it would be a good thing to learn (or re-learn) how to read standard musical notation, but a lot of mandolin music is available in tablature (or tab), which is a much easier system to use and doesn't require the ability to read music. The Julin book I linked to above uses both.

Edit 2: Also, here's a recent thread on the Rogue, just so you can see some other people's opinions.