Don't let yourself get cold by standing around after you have finished. Make sure you keep moving, especially if you feel dizzy, and have something to drink. Collect your bag as soon as you can and change into warm, dry clothing - foil blankets won't stop you from getting cold. Then go to wherever you have arranged to meet your friends and family.
Follow the advice on this website about eating and drinking after the race. If you haven't drunk enough or eaten anything, you can feel faint even over half an hour after finishing a race. If this happens, have something to eat and drink, but don't drink too much.