To make sure that your folds are sharp and accurate you'll need to score them. Scoring is the process of making a dent in the card where you want to the fold to be placed.
There is no one specific tool for scoring, the point of a pair of scissors works well, so long as the scissors aren't too sharp. You could also use an empty ballpoint pen or the end of a metal nail file.
Run the point of your chosen instrument along the line you want to score. Press down but make sure you don't cut through the card. Use a ruler if you are scoring a straight line.