Usually, it is possible to play with play money in casinos with games of Merkur. However, you cannot win real money this way. Better look for offers with free spins.
Casinos that present the games of Merkur are entirely without a doubt reputable. The mentioned software stands for this seriousness. Other indications are the license of the casino and whether, for example, payments via PayPal are possible.
Usually, it is sufficient if you are 18 years old, i.e., of legal age. And you must not already have an account in the respective casino, because registration is only possible once.