If you are completely new then better visit most popular casinos out there since it will help you stay away from the scam sites out there. There are plenty of names such as Roobet, Stake, Duelbits, Primedice and much more in the bucket list. Visit their official ANN listed on the forum (in the current section) and you can get hang of it after reading in depth. You can also visit freebitcoin if your core focus is on sports betting. They have simple betting formula, simple UI and plenty of games to choose from. Always have a habit to read about their TOS though they are boring long text they can save yourself from complications in later part. Good luck mate.
Why only new gamblers , I suggest even if you are old experienced gamblers you should stick to these trusted websites that are also active on this forum and play / bet on whichever sites among these suits you the best. Sometime you will not find all the games to bet on , all all the sites so i prefer stake.com when i had to bet on any match.
Another thing you can compare between the sites is the odds they offer for the betting. Some sites give better odds than others, a careful analysis will let you choose the best one for you.