To warm up, start with some dynamic stretching, or stretches that incorporate movement. These might be butt kicks, high knees, and high jumps. ou can also incorporate some side to side leg swings. Static stretching, or stretching cold muscles without first warming them up at all, can quickly lead to a nasty injury so avoid it if you can. Start by walking and then slowly increase your pace and begin incorporating some striders, which involve a slight lengthening of your stride and increase in pace. Don't do too many striders, just enough to get the blood flowing then gradually decrease your speed again back down to a walk.
Then, depending on what kind of races or running you will be doing that day, start out with a nice slow jog that is longer than the length of the field or area that you were doing your striders in. Again, do not get your heart rate too high, but focus on making sure your form is good (90 degree elbow bent, relaxed neck, shoulders, and hands, and a comfortable stride length).
Finally, ask most runners and they will tell you that running is actually much more of a mental sport than it is physical. Running is hard. We all know that it is not always the most comfortable thing in the world, and that there are certainly days we would rather just change into our pajamas and crawl back into bed or curl up on the couch with a pint of ice cream than go for a run. Running requires an immense amount of mental strength to push through the tough parts. So you really can't start out by telling yourself you're going to hate running, because then you probably will! Even if you think that you do not like running as much as other people, go into each run (or in your case, each class) with the mindset that running is going to make you a stronger person, give you endorphins and improve your mood, and maybe lead you to a group of friends you wouldn't have otherwise had! You might actually end up loving it!