The most important thing for a beginner is to stay consistent with your exercise. This is rule number one. Make sure you are walking and or running four times a week. This might seem like a lot, so maybe start with three times a week, but you've got to be consistent about doing this every week. I suggest running and walking for 30 minutes each time you get out there to exercise.
Rule number two is to increase the amount of your running from week to week. Maybe you start off running for 10 minutes and then walk 20 minutes. Try upping it to running for 12 minutes and then walking for 18. Each week try to add more running and cut back on the walking time. There will be set backs, but that is very normal. Just be consistent and go out there and try your best the next time.
Those are my two very important tips for 5k training. Now get out there!