I maintain that Chinese rules are easier to understand for a beginner. Calculating score by quickly scanning the board is easier, and the concept of not filling in your own territory is difficult for beginners to understand. I have also found my students do better with Chinese rules in the beginning than they do with Japanese. I discuss beginner perceptions at great length in my guide: What Are The Differences Between Japanese, Chinese, & New Zealand Rules?
For these reasons, I think Chinese rules are a better place to start. That is my personal opinion and has been my experience. I would like to point out that New Zealand are my favorite rules on OGS and that I’ve run a New Zealand Tournament Group for close to a year. Point of brining that up is that I do, very much, drink my own kool-aid 
To be clear, there is nothing wrong with Japanese rules. The concepts of scoring are just a bit harder to wrap your head around when you are just starting out. But choosing them will not put you at a disadvantage or lead to anything negative. I am not advising you to steer clear of Japanese rules, though I am whole heartedly endorsing Chinese and New Zealand rules. To better understand the difference between these two, you can read more here: Are There Legitimate Reasons To Choose New Zealand Over Chinese?
There are a few different places where you will see which ruleset is being used. Within an active match you will find this in the Game Information panel.
You can access this panel from collapsible Side Panel during a match.
The other place you will see the game ruleset being referenced is where you go find or create Go matches. For matches that are open awaiting a player to join, it is the last column of information.
On this same page you can choose to create a custom game. After clicking on that button, you will have the option of selecting the ruleset you desire for the custom match.
On the same page where you can create a custom game, there is also a settings button for Quick Match Finder. Opening that up will reveal an area where you can select which ruleset you prefer to use.
Finally, when someone sends you a challenge, it appears on your Home page. In this window, it lists which ruleset the match utilizes.
When you receive a challenge, it will produce a notification. When you view that notification menu, it will also list the ruleset being used.
I think that is all of them 