You should drink water before, during and after your exercise, this will keep your muscles hydrated and less likely to go like jelly while exercising, particularly so soon into the workout.
Regarding the feeling sick part of it, are you eating heavy meals? It's recommended that you eat something light about an hour to 90 minutes prior to exercise. Another reason for it could be you're not breathing properly during exercise and the oxygen is essential to the muscles (could be part of the reason you're like jelly).
So, light meal an hour or so before commencment of exercise, plenty water (preferably throughout the day too, not just exercise time), and be aware of your breathing.
Keep with it, both symptoms do pass as you get fitter.