First time I put my finger in I found it.
Some ideas to try. First, make sure you are fully aroused and good and hard. When you are the prostate engorges and that makes it much easier to find. You are feeling for a bump/ball about the size of a walnut about 1-2" inside on the front wall (towards your belly) of the rectum. The closer you get to climax the bigger it will get, so that may help too.
I found the best way to stimulate it in the beginning is with my middle finger and use a "come here" motion. If you have a partner that can help that is even better.