I wish to buy cryptocurrency (Monero) for the first time: I have no real knowledge of how to do this, and I wish to pay in cash. Can this be done? I have bought a Ledger Nano S but haven't received it yet. Could someone spell out step by step how I do this without getting ripped off? Is Monero a good choice for investment?
You can purchase Monero for cash directly on sites like https://localmonero.co/. Alternatively buy BTC for cash somewhere like https://localbitcoins.com/ and convert to BTC to XMR on something like Bisq or ShapeShift.
You specifically stated "pay in cash" which is why I didn't list alternatives.
To store on a Ledger, you will need to set this up first (either using the Monero GUI or CLI) and use your new wallet address as your receiving wallet when you buy your XMR. A GUI guide can be found here and CLI here.
Your subsidiary question is “Is Monero a good choice for investment?”, and you are presumably using “investment” to mean either “trading” or “speculation”. Monero, like $, ¥, £, ฿, €, ₦ etc. doesn't produce goods or services so the word “investment” doesn't apply. I will therefore comment on trading and speculation.
Monero is newer than $, ¥, £, ฿, €,₦ etc., so its exchange rates are more volatile, and a skilled trader can make money out of exchange rate volatility. At its simplest this means buying at the troughs and selling at the peaks, while more sophisticated trading techniques (e.g. short selling) are used to profit while prices are falling. However, very few people have the skill and self-discipline to profit from trading in the long run. Bad short selling strategies can be ruinously expensive.
If you meant “speculation”, then you're effectively asking “Will Monero be worth more at some point in the future than it's worth now?” If you had asked that question in 2016, and your “point in the future” was now (Aug 2018), then the answer would have been Yes. But if you had asked the same question in Jan 2018, then the answer would have been No. In either case, the answer would have been determined more by market irrationality than by fundamentals. A simplistic fundamental analysis says Monero should be worth more now than it was in the past because more man-hours have gone into improving the technology (of both the currency itself and its supporting infrastructure). Monero's purpose is safe, decentralized and fungible value-transfer, so if you speculate that its value will go up, you're really speculating (i) that there's a growing demand for safe, decentralized and fungible value-transfer and (ii) that Monero (rather than some alternative project) will prove to be the best technology for this purpose. One disadvantage of using Monero for speculation is that there are no derivatives available to hedge your position.