What is the best move in this situation? This is my first post on CSE, so I appreciate any guidance on how to frame my questions from now on. 

Here is my question: I want to know what the best move is in this sitiation.

  [FEN ""]

1.e4 c5 2.d4 e6 3.d5 ?


I was playing Black, and I won this game, but by moving my pawn to d5. a very dangerous situation will be created that can be irritating until the end of the game. 

I understand that we should not allow such scenarios, but my specific questions are:


How to defend in such scenarios theoretically?, and on the other hand,
What lines could White play to win this game (if such a thing is even possible)?
but by moving pawn to d5 a very dangerous situation will be created that can be irritating until the end of the game.


It can be dangerous to both sides. Positions of these type are generally characterized as the Benoni Defense. They are unbalanced and offer more chances for complicated play. So you may or may not want to avoid such positions depending upon how comfortable you are playing such positions. 


  How to defend in such scenarios theoretically?


A good idea for Black is to immediately take on d5, followed by moving the pawn to d6, blockading the pawn on d5. A sample line of development for Black may go like this - 

  [FEN ""]

  1. e4 c5 2. d4 e6 3. d5 exd5 4. exd5 d6 5. Nf3 Nf6 6. Nc3 Be7 7. Be2 O-O 
  8. O-O Na6 9. Re1 Nc7 10. a4 b6 11. Bc4 a6 


The strongest player to consistently play this opening successfully was Bent Larsen. I recommend you take a look at the following games of Larsen to see how Black not only defends this position but also manages to seize the initiative. 

Alberic O'Kelly de Galway vs Bent Larsen 0-1 1967

Svetozar Gligoric vs Bent Larsen 1/2-1/2 1969 

Wolfgang Unzicker vs Bent Larsen 0-1 1970

Miguel A Quinteros vs Bent Larsen 0-1 1974 

Incidentally, even Garry Kasparov once tried this idea in his early days. 

Yuri Balashov vs Garry Kasparov 1/2-1/2 1979


  What track could white take to win this game almost surely (if such a thing is even possible)?


It is not known yet whether White can forcibly win in this line. Strong players continue to use this line, although it is not popular at the highest level; but that might simply be because of reasons like taste rather than the objective quality of the position.