In short: the best online casinos are casinos where you as a player can enjoy your casino game safely. This means: casinos with the right licenses, good and clear terms and conditions and proper support. But there is more to a good casino.
To make your deposit at an online casino you can use various payment methods, including credit card, online bank transfer and many online alternatives. Make sure you actually have spare money to play with and avoid getting into debts.
No, playing offline means you cannot play casino games for real money. Of course you can buy or download some casinogames, but if you want to try your luck with some real cash, you need to register and play online.