The layers could be where you are getting stuck. One layer might be selected and you are trying to draw / edit on a different layer. But some questions are: Are you trying to apply an effect? Which paint tool are you using that you are having the problem with (fill, brush, selection tool etc)?
You have to imagine you are asking a blind person for help. We cant see what you are working with or doing. So we have to ask questions and at times take a good guess. If you could upload the file some place so we can have a look and then you can describe what you want to do, we could take you thru the steps.