Congratulations! While the role you are about to take on is difficult, it can be extremely rewarding. Obviously, the better you get to know your future stepchildren, the better you will fare in bonding with them. So first off, spend time with them. Engage them in conversation, attend their school events, do chores with them. Secondly, discuss with your future spouse your role in the stepfamily. It will likely vary depending upon whether you are a custodial or non-custodial stepparent. How will you contribute financially to your stepchildren's needs? Will you be involved in discipline? Thirdly, relax your expectations. I bet you didn't grow up planning to be a stepparent! But the expectations that are swimming in your head are preconceived notions of what a stepparent should look like. However, it is important that you be you. It is too time-consuming and unproductive to be someone or something that you are not.
It is important to keep in mind that your future spouse loves you enough to bring you into his family and be a stepmother to his children. When in doubt, lean on him. He knows his children and will help guide you through the parenting process.