Can someone please help me in writing an essay on Sir Gawain and King Arthur? I always seem to get an F on essays. Thank you.

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!