This is a common problem in the beginning...
Try to warm up by dipping your wiener into a glass of very warm water instead of using wash cloth. This way you won't have any friction to get erect.
Anyway,even if this does work, you'll probably get erect during exercises...
So the solution should be this: masturbate and ejaculate prior to doing PE. This way your penis will be "satisfied" and it should give you any problems.
Hope this helps.