Hey @Dkprime,
I believe as a new user you will not have any issues getting into Stadia as it's working pretty simple. A big bonus in comparison to the Play Station you've used before is that you don't need to download or update games. You simply select your game, click Play and your adventure begins.
So first of all, depending on your country, you could use Stadia immediately on a PC/Laptop via stadia.google.com (Chrome Browser works probably best with it, but other browsers can work too) or with the Stadia app on a mobile device. To play on TV, you would need a Chromecast + Stadia controller, which you can get in the Stadia Store for example. Here you can learn more about requirements. Stadia works well if you have a good internet connection, you can check this with this speedtest if your internet is good enough to play Stadia with it. Of course, the better it is, the better also qualitywise. 
Usually when you use Stadia for the first time, it's offering you 1 month Stadia Pro for free (free trial) and after that you would be charged with the monthly cost of the Stadia Pro subscription. The charge is not going to happen though when you cancel the free trial before the end, so if you just want to try out Stadia Pro, you could give it a try. This allows you to claim some games immediately and play Stadia on highest quality. You can learn more about Stadia Pro here and the difference to using Stadia without it. So as you can see, you don't need to have the monthly subscription, it's up to you. If you don't want to try the free trial for 1 month at all, you can simply skip it by opening this link to Destiny 2. Destiny 2 is an example for a game that you can play on Stadia without paying anything as it's free for everyone.
While Stadia Pro offers you game discounts and some games to claim for free, you can also decide to only pay for games you want on the platform and just play those. Once you purchase a game, it's yours and you can play it always, no matter if you have Stadia Pro or not. When you claim games for free you can only play them as long as you maintain the subscription. There are discounts as well sometimes for all games, even without Stadia Pro, just in case you want to wait for offers.
Mhhh I can't think of anything else you really need to know before starting, but I can recommend you to browse in the Stadia helpcenter. This should also answer some more questions around Stadia that you might have. Here is a Stadia quick start guide that might help you too.
