Are you primary or secondary? What subject?
Can you have a browse on TES to see if you can look at any example lessons for the subject? Might give you some ideas?
You need to start with the lesson objectives - what do you want the pupils to be able to do?
So, as an example, an objective might be:
To be able to calculate the weight of an object. (Sorry, I'm a scientist so this objective may not mean much to you).
Then think about what needs to be done to allow the children to meet that objective.
So, for my example above, I would need to explain what weight is and show them the formula.
Then, I would model how to use the formula.
Then I would likely get them to do a couple of practice questions on mini whiteboards so I can assess who has got it and who needs more guidance.
Then an opportunity for them to practice - could be a written activity or a practical activity. I might assess while they work giving feedback or we might self/peer assess.
Finally, a plenary - might be an exam question or a quiz of some sort.
Once you have this, consider how you will differentiate your lesson.
So, I might have scaffolded practice questions for those who need support. I might have questions requiring pupils to rearrange the formula or convert between units for the more able.
Has your mentor done any joint planning with you?