I’ll first start with a warning that many online casinos do not accept players from the United States because of the many regulations, so look out for that when deciding on an online casino to play at.
Considering that you will be playing with your own money, verifying any site’s safety should be your first concern. The top online casinos are independently audited and regulated and will support the top notch SSL security encryption, so you know your bank information will be totally safe and secure. Of course you want to be comfortable and assured that your winnings will not be hacked away by some idiot. 
You want to know that you would be able to collect any winnings. The site may let you make a deposit using one method, like paypal, but in order to withdraw any winnings, another method woud have to be used. That kind of information is usually found within the FAQ section of a website. Make sure you know all the money details before you place your first bet.
Check for published lists of bad, as well as, top-rated casinos. A top-rated casino will be certified by a reputable third-party governing body, like the eCOGRA (eCommerce Online Gaming Regulation and Assurnce) or Technical Systems Testing (TST).
Once you figure out the site is secure, you want to look at the bonuses the casino offers. Online bonuses usually come with terms and conditions, including bet requirements to eventually withdraw any winnings, the specific games you must play to use the bonus money and whether a specific amount must be deposited to receive the bonus money. You do want to pick the largest online bonus, keeping in mind the specific terms and conditions. You can find sites online that will rank the top online blackjack sites, offering the biggest bonuses. As with promotions anywhere, “free money” may not really be free, read all the fine print.
Another key factor to look for in online casinos is that they use Random Number Generators. This is what determines the shuffling and dealing of cards for fair play. The top sites will show that their RNGs are regularly audited and tested as well. 
Last but not least, is to determine that the site has a good variety of blackjack games to play. The different varieties let a player make money from a variety of a side bets, keeping the player challenged and interested.
Blackjack online provides greater flexibility and unique features not offered at actual casinos. Playing online can be less stressful, with all kinds of practice games and tutorials available for learning. With continuously improved technology, the once perceptable difference between online and actual casino blackjack has decreased significantly to the point where blackjack online is beginning to over take land-based playing. You can’t beat the convenience of being able to play at home.