Industry language
Firstly, it's a good idea to use words that people in your industry and role will recognise. Also, try and keep them concrete, i.e. not too abstract.
Think about a recent challenge you overcame
If you are struggling to work out what they are, then think about a challenge you recently faced at work (or even personally) and how you went about dealing with it. What skills and experience did you draw upon in overcoming the challenge?
Ask others
Maybe ask your friends, colleagues or co-workers to tell you what they think your strengths are? They often come up with things that you might not have thought of.
Finally, test the strengths you came up with
Once you have come up with some, you can test them out a bit by:
Asking yourself where have your strengths been used
Asking yourself if someone giving you a reference or testimonial would be willing to confirm them
Asking your friends or family what they think