You can compare something with another thing or contrast something with another thing (this can be person or object too) or you can both compare and contrast in your essay. There are two patterns for this kind of essay; block by block and point by point.
Block by block: You compare or contrast the things in single paragraphs. For example, in your first paragraph you focused on the colour of lemon. You told everything about its colour. You told us that it is yellow. In your second paragraph you say to us that though lemon and orange belongs to the same family, orange has a different colour and you go on like this.
Point by point: In this type of style, you compare or contrast things in sentences subsequently. For example, Ali is a good boy, he can help you. On the other hand, Ayşe is not as a good person as Ali, she may refuse to help.
I don't sugges you to write a compare and contrast essay. It will be long and complicated and I don't know the style of this kind of essay.
Don't forget one thing: if you compare something don't write any difference between the things or if you contrast something, don't write any similarities in your sentences or paragraphs.