The second section of SIr Gawain and the Green Knight (trans. Jessie L. Weston) opens with a description of Camelot:
King Arthur lay at Camelot upon a Christmas-tide, with many a gallant lord and lovely lady, and all the noble brotherhood of the Round Table. There they held rich revels with gay talk and jest; one while they would ride forth to joust and tourney, and again back to the court to make carols; for there was the feast holden fifteen days with all the mirth that men could devise, song and glee, glorious to hear, in the daytime, and dancing at night. Halls and chambers were crowded with noble guests, the bravest of knights and the loveliest of ladies, and Arthur himself was the comeliest king that ever held a court. For all this fair folk were in their youth, the fairest and most fortunate under heaven, and the king himself of such fame that it were hard now to name so valiant a hero.
If Camelot is characterized by "rich revels with gay talk and jest" (in other words, lots of joking around, happiness, and all that), what is Heorot like? Are people in that court laughing, joking loudly, and partying still? They aren't, of course, and the reason is Grendel, who's particularly sensitive to and offended by the sound of merrymaking.
Other points of comparison and contrast might be the presence and role of women as well as the use of competitions (e.g. jousting or storytelling) and the heroic challenge.
I would suggest calling the two places "courts" rather than "mead halls." Heorot is certainly a mead hall, but Camelot is not (as far as I know).
What you should focus on first as you being to lay out the plan for this assignment is your pattern of development. You could do a point by point comparison (looking at the physical descriptions of each, the people who are present in each, and finally the tone or mood of each) or you could examine one and then the other.
Once you have determined the pattern that you will use, I suggest that you determine what most makes these two places similar--try listing or mapping for this. A Venn diagram might be useful for you. Is there a primary similarity that you can clearly identify? This might be a good starting point. Then, ask yourself if there is a primary difference. In this case, I suggest as the prior respondent noted that you look to the mood of the scene. In Camelot, there is joy and happiness. In Heorot there is fear and trepidation. In what way does this difference impact the mood of the work as a whole? Is there a sense of foreshadowing, perhaps, in the details of the setting?
In any case, start with some pre-writing and note all of the details that you can see about each one. Look for areas of overlap and areas of divergence. This should help you!