I would have to agree that finding the contrasts in these two works of art will likely be quite easy to accomplish. As far making comparisons you are going to have to look at the obvious ones, as mentioned above. You might also look at the circumstances of the two artists and make some comparisons.
Well, in looking at both Rembrandt's Storm on the Sea of Galilee and Renoir's The Skiff, it's clear there are more contrasts than comparisons between the two. Let's start with the obvious comparisons. Both depict a body of water, a boat of some kind, and bodies (people) in the sailing vessels. Both works are painted with oil on canvas by painters of great renown. I think if I were writing I'd start with that, adding any other similarities you may have discovered. Then, of course, I'd go on to discuss how the two works are drastically different.
First, they're dramatically different in tone (emotion). One is dark and dangerous; the other is calm and serene. One has no other land visible, implying a vast ocean with little hope of reaching land anytime soon; one depicts the backdrop of nearby land, enhancing the safety and serenity of this outing. One depicts a group of sailors clinging to whatever they can to keep from being thrown overboard; the other shows only two passengers in the boat, and neither appears to be frightened in the least. One shows a violent sea in which neither this ship nor the men on it have any control; the other shows a perfectly peaceful body of water and humans who are in control of their craft. And the list can go on.
Those are some key differences in subject matter and imagery, and I'm confident you can find many more as you start looking. What I haven't discussed here is the difference in painting style. I presume you're in some kind of art history class and you will be able to spend some time on that yourself. It's not so very difficult to start writing, I think, if you first discuss any similarities between the works and then move to the more pronounced differences or contrasts.
One other way of writing a compare/contrast essay is to write everything about one painting, then write everything about the other--being careful to discuss each element in the same order for each paragraph. (In other words, go point by point through one painting, then use those same points in the same order to discuss the other painting.) This automatically sets up a comparison as well as a contrast, if you use effective transitions. (For example, "In contrast to Rembrandt's darker color palette, Renoir uses bright colors and plenty of light." Or whatever point you're making.) Just relax and start; I think you'll find it's not as difficult as you think.