The criteria for being “correct” is relatively simple:
- Do you have at least two motives. A motive is a short melodic idea that usually has some kind of characteristic element. Often the first motive has an opening up shape – like moving upward, and the 2nd motive has a closing down shape – moving downward – but any shape works. Multiple motives help as they give you something to split up or fragment.
- Harmony should prolong tonic. This means that your harmony should start on tonic, and either move back tonic before the end of the basic idea, or move back to tonic by the start of the repetition or contrasting idea.
If you do these, the basic idea will generally be “correct”. But this doesn’t mean “good”. That is a judgement call. If you feel its a good basic idea, then you should move forward with it and turn it into a theme. It doesn’t really matter if other people think its a good basic idea or not. The more important thing is to practice using the basic idea to write themes and complete pieces.
I recommend looking at a bunch of Beethoven piano pieces to see how he writes his basic ideas on themes. The bagatelles are usually really easy to understand. The piano sonatas can be a little trickier, because he often starts with slow introductions, so you have to hunt around for the basic idea.