listen up mate you say I want texas holdem blablabla
you dont really know what you even want. so you need:
1. software analysts to figure out what is and is NOT to be done.
1.1. First of all the rules of the game has to be settled. Texas holdem has many variants fixed/no limit/tourney/sattelite whatever, also details like do the players change seats, do they reseated when fold, etc.
1.1.1 game guarantees like how you guarantee server latency and ddos resistance
1.2.If you want multiple variants or full casino, which games and which not. Make the software DIFFICULT TO ADD future additions makes the project significantly cheaper & faster & easier to develop.
1.3. Details of the infrastructure. Where is served public cloud/private servers etc. How will you guarantee users don't see each other cards. How will you guarantee the cards are dealed randomly. What to do in case it fails etc.
1.4. Details of the payment system & its security. What to do in case of breaches.
2. implement the servers software. start with login/auth/payments deposits,withdrawal. next comes the management of game sessions/rooms/turneys/whatever./next the dealing of cards/logging them to a safe place/doing the actual game actions fold/check/raise/computing the winning card/logging/making the desired accounting changes based on the outcome/game quit
3. now come the time for the sugar. you hire artists to make a cool gui in flash/html5 whatever. you add crap features like avatars and whatnot.
4. testing. you hire or rent someone who can test an application. after many weeks adjusting 2. and 3. you maybe get a working application.
5. hire an engineers who review your infrastructure and determine what needs to be reworked in other to guarantee latency / availabiliry. skip this and turn of users playing for $10000 expires yet have fun explaining them on tech support they didn't BET and their pcs are bad when in fact your game is laggy. Also have fun when somebody DDOS your casino and it vanishes from the internet during an important truney finals.
6.after several months you MAYBE have a secure application. or not and user's (in fact YOURS) coins simply vanish. oops. wait i know the answer: YOU steal them earlier than they
welcome to the world of software developement .