Your question is a bit vague, but I see you're still waiting on an answer so I'll share a few ideas with you which may get you started on the right path, anyway.
You mention two characters, Sir Gawain and King Arthur, and I assume you're interested in comparing and contrasting them. When you don't know what else to do, focus your thoughts by making a list. List the qualities and characteristics of each man, the actions of each man, and whatever else you might know about each of them. Now compare lists. The elements they share become the points of comparison, the areas in which they diverge become the points of contrast. Pick two or three of each, depending on the required length of your essay, and those are your body points.
If it's simply an informative or expository essay, your job is even easier; simply pick several characteristics which you know or have researched about each and write kind of "matching" paragraphs. In other words, if you discuss, say, Gawain's life, deeds, and disgraces, you should turn around and do roughly the same thing in the next section with Arthur.
Next write a strong, clear statement of purpose (thesis statement) which includes the three points you'll be writing about in your essay. I've included a link below in case you need some further help.
Every good essay begins with an effective introduction. You might find an interesting quote or a short story or whatever, something to capture the readers' interest and let them know the direction in which you'll be heading. Make certain you have a strong purpose statement, as well. The conclusion should both summaraize your points and leave the reader satisfied.
You mention you don't seem to be doing very well on your essays. I have a few suggestions which might help. First, I had to correct your spelling and punctuation in the question you asked; that tells me perhaps you're making too many errors in your essays, as well. It's always a good idea to have someone you trust read through your essay at least once to proofread for errors. Make sure you do the same. That's an easy way to improve an written work.
Second, ask your teacher for some ideas on how you can improve. I know that writing is not every student's strength, so if one of my students comes and asks me how to get better at it, I will go out of my way to offer help. I'm confident your teacher will do the same. Best of luck!