I don't understand how to buy PIVX.
Definitely read up and learn what you're getting yourself into, but if you want the TL;DR version....
This applies to pretty well any coin out there. Make an account with Bittrex or Cryptopia (or whatever exchange you like, but those are your best bests for PIVX). Find your BTC wallet address in whatever exchange you join. Send BTC to that address. Go to the BTC-PIVX market of your chosen exchange, and place a bid for X PIVX at Y price. If you don't care what you pay, you can make an immediate purchase at whatever the current asking price is. If you want to bid below the current asking price, your order will stay until it is filled--what you bid depends on what you think the price of PIVX is going to do going forward. Once purchased, your PIVX will be in your PIVX wallet at the exchange. If you want to sell again soon you can leave coins at the exchange, but you will not be able to stake them. Generally, you should withdraw your coins to your own PIVX wallet, which you'll have to download and install via the OP or PIVX website. Once the wallet is installed on your computer, withdraw your coins from your exchange wallet to the address of your own wallet. They should appear in seconds. Make sure to encrypt your wallet using a strong passphrase (under 'Settings'), and then unlock your wallet 'for staking etc. only'--this will allow your coins to stake, so long as you leave the wallet running. Note that some people are having occasional crashes with the most recent wallet, so be prepared for that or wait a little for the next release. The Devs are very efficient and usually fix these kinds of bugs quickly. (Note that a wallet crash doesn't mean your coins are lost; just that the wallet program acts up sometimes. It might be scary for a first-time user, but it's not usually a big deal.)
Good luck.