You'll need to be old enough to participate in real money games at a casino in your country or state of residence. You'll also need to register for an account. And if you want to play for real cash you'll be required to deposit funds into your casino account using one of the accepted deposit methods. If the casino has offered you a welcome bonus for joining you'll also need to ensure you can meet the playthrough requirements in order to claim it.
Easy. Just visit the website of the casino you want to open an account with. You'll find the option to register on the home page and you'll just have to supply a few basic details about yourself in order to open an account. Once registered you'll need to link a deposit method to your account so you can transfer funds to play with.
It depends on your country of residence. In the UK it's 18 years old. The U.S has different age restrictions from state to state but it's generally 18 or 21. If you're unsure you're acting within the law it's best to check the legal gambling age in your state or native country before joining an online casino.