Here are some links to articles, which could help as starting points: Poker Logic in C# and Fast, Texas Holdem Hand Evaluation and Analysis
“This code snippet will let you calculate poker probabilities the hard way, using C# and .NET.”
The theoretical fundamentals are given in this Wikipedia article about Poker Probabilities and in this excellent statistical tutorial.
An example of a complete project written in Objective-C, Java, C/C++ or Python is found at SpecialKEval. Further links and reading can be found therein.