This is an issue with stimulus control. It's pretty common for a dog new to clicker training to start throwing behaviors at the handler in hopes of guessing the right answer and getting a cookie.
This is a good problem to have, insofar as it shows that the dog is interested in working with you and motivated to try things. It's also very easy to fix with clarity, consistency, and practice.
Couple of discussions on the topic: Everything You Wanted to Know About Proofing?But Were Afraid to Ask | Karen Pryor Clicker Training
(the whole discussion on fluency is worth reading, but you can scroll down to the section about "Stimulus Control" if you want), see also Stimulus Control: The Most Important Concept You’ve Never Heard Of
Those links should give you a clearer understanding of the issue and, hopefully, some ideas about how to solve it. Normally what I do is set up discrimination drills: once I have a couple of clearly defined behaviors that I know the dog understands and can do on cue, I'll alternate asking for them and also not asking for anything (in which case the dog gets clicked and rewarded for not
doing anything besides maintaining attention and waiting for my response).
After the first couple of sessions, if you're not seeing improvement, then the problem is probably on your end -- something in your vocal cues, intonation, and/or body language is confusing your dog, so figure out what that could be and fix it.