How old/tall/heavy are you?
If you're brand new to cardio/aerobic activity, you will most likely be surprised at how little tolerance your body has for these types of activities.
First off, I would just recommend testing your tolerance. A good start is just to set a distance and a time that will be your goal. Your best bet is to warm up at about 2-3MPH for 5-10 minutes, and start jogging at a pace that seems acceptable to you. Start somewhere between 4-8MPH and see if you can make it to a mile. Go as long as you can with your walk/jog, but do not stop until you reach that mile marker. If you need to slow down, that's fine, but don't stop.
The key is to keep the blood flowing and your heart pumping. When you are comfortable doing the mile at that pace, you have several different options: If you're going for endurance, you may want to stay at that pace and try running another mile or two. If you are trying to get some speed in that mile, add some speed and see how fast you can do the mile.
If you're trying to lose weight, your best bet is to include HIIT of some sort, but before you even think about doing HIIT, please make sure that your body is conditioned enough to handle it. Before starting HIIT, you probably need steady cardio 4-6 days a week for 6-8 weeks to avoid injury.
Oh, also - when running, it helps to get a breathing pattern. When running, you should breathe with every step as a count. For example, I breathe on a 3-2 pattern, which is breathe in for 3 steps and breathe out for 2 steps. Make sense?