Buying and selling XSPEC works the same as for any other 'alternative', i.e., non-Bitcoin, cryptocurrency. You need to convert your fiat currency of choice (USD/Euro etc.) into BTC on an exchange. Which one you choose will usually depend on your country of residence, and you'll find lots of opinions online about which ones are best. You may need to verify your account with ID. Once it's set up and you have money on the exchange, you will make an order for X BTC at Y price per unit. You can either pay the current asking price, in which case your order will be filled more or less immediately, or you can place a buy order at a lower price in the hopes that the price will come down. In the latter case, your order will remain open until it is filled, which could be in an hour, a day, a month, or never.

Once you have BTC, you will need to use that to buy XSPEC. Because XSPEC is only listed on small exchanges, this means that you will need to make an account with an exchange that trades XSPEC and send your BTC there. For XSPEC, your best bet is Cryptopia (there is also Bisq, but that is arguably more challenging for a newbie: https://bisq.network/ I will assume you go with a 'normal' exchange, but obviously up to you.). Once you have an account, you'll need to find the deposit address for BTC, and send BTC to it. Once you have BTC in your Cryptopia account, you will place an order for X XSPEC at Y price in BTC per unit, again with the same basic parameters as for the previous example. As an aside, note that there will be fees for every step in this process: the actual trades, and also the act of transferring from one place to another. Make sure you factor this into your math.

So now you have XSPEC. The next step is to download and install the wallet on a secure computer in your possession, and wait for it to sync (you will likely need to add nodes manually to get it to sync (https://github.com/bitcoinx2/spectre/wiki/FAQ#7-i-have-zero-connections)). This will take a few hours. Once it's synced, send your XSPEC from the exchange to the address of your new wallet. Send e.g. 0.1 XSPEC as a test first, and once you have confirmed that it's working, send the rest.

After that, you'll want to encrypt your wallet with a secure password, and consider leaving it running 24/7 to stake a receive occasional 'interest' (the cost-benefit ratio of doing this will depend on how many XSPEC you have. If you have free electricity then it doesn't matter; otherwise, you'd want probably north of 500 XSPEC for it to begin to be worthwhile). That's it.

More info and a step-by-step guide here: https://medium.com/@spacemanmatt/how-to-stake-spectrecoin-xspec-5f533c73ce52