If you don't run at all at present you need to start now, but should stay with short distances to begin with and don't try to go too fast. If you can manage 15 to 20 minutes for your first run that is a good start. If you have to stop for a moment, just get your breath and go on again - the main thing is not to give up. After you've done a few short runs (try and do 2 or 3 a week to build up your fitness) you will be able to keep going for the whole time.
Once you've reached this point you can start to make a real schedule, which should include aiming to do one longer run a week as well as a couple of short ones. You can get a good idea for a schedule here - scroll down to band 3 if you are a beginner or haven't taken exercise for a long time. It's also important to stretch before and after running to prevent injury.