Getting started with kadokado is super simple and easy. Just create an account and then choose the kinds of games that you like to play. There are two broad categories of games that you can choose from, action and puzzle. Really the style of games blur the lines because some of the action style games play more like puzzle games and vice versa. But there is guaranteed to be a game that you like on kadokado.
What about cost? If you want to rack up the kadokado points then yes, you will have to pay money to get more diamonds, but there is no real need to do that if you only plan to play once in a while. kadokado gives you 5 free diamonds everyday that you can use to play any games of your choice and by doing this you should be able to get 1000 or so points every week for free which will let you unlock new games.