One of the most unique features about Steem Monsters is that this game runs entirely on a blockchain instead of using traditional servers. This means that you don't really need a "Steem Monsters account", but can instead just use any account on the Steem blockchain (a "Steem account"). This means that you can obtain the account from any Dapp that gives away or sell Steem accounts, and you don't need to get it directly from Steem Monsters.
That said, one of the easiest ways to get a Steem Monsters account is to create one from the Steem Monsters website. On the main website, click on the "Get in the game" banner to be taken to the account registration menu, and you will be promted to buy a Starter Set for $10. You will need to buy this Starter Set even if you have a Steem account from another location, so you might as well buy it directly.
During the process of buying the Starter Set, you will be asked if you have a Steem account already. If you don't, now is the time to create one. Steem Monsters will create it for you as part of this process, and it will be ready to use right after you have bought the Starter Set.
You can buy the Steem Monsters Starter Set and account with pretty much any crypto, with a Promo Code that they give away every once in a while, or with credit card. The last option is great for those of you who don't really know much about blokchains yet.