[sentence]。すると、[sentence]
すると starts a sentence and means "and then", "whereupon": you did something, and this is what happened next. It links two sentences, and the second one tells what followed, usually something you saw or found rather than chose to do.
The sentence after すると is outside your control: a result, a discovery, a surprise. For your own next action, use それから or そして.
In conversation すると also opens a conclusion drawn from what the other person has just said: "so then", "in that case". The sentence is often a question checking that you have understood.