Resident Runner coming through
Basically you need to increase the amount you run slowly and steadily starting from about a minute or two at a time (at a guess I'd say that you're currently doing more than that if you cover 400 metres) up to half an hour or 5k. Once you can do half an hour/5k you're into a whole new world of training schedules, but let's not run before we can... erm... run
For beginners 5k isn't always 30 minutes, although it's a handy number to bear in mind - you'll start off slower than that but with regular running you'll almost certainly be able to run it quicker, and it's a nice target to aim to break through.
There are two basic approaches I've seen. One is that one day you go out for a one minute run. That's it, nothing more than that. You then go home. The next day you go out for a two minute run. Then three. Then four. And so on for the next 30 days (although I'd build in some rest days if I was doing that plan). I like it because it's very simple and the first few days involve relatively little exercise (I'd also try to build this into the rest of your exercise programme particularly at first so that you're not just exercising for a minute or two each day). This is simpler to describe, and probably works, but you might feel like you want to build it into a longer workout so you're not getting changed into running kit for a minute or two. I also tend to think that an interval plan is slightly better than this.
The second approach (the best known version is couch to 5k, although a lot of the beginners 5k plans are based on the same principle, there's probably one on the Race for Life website and one on Runners World), is to have roughly a 30 minute run/walk every time you go out, but to increase the amount of time you spend running during that by using run/walk intervals. So the first time you go out you might run 1 minute, walk 9 and repeat 3 times to get to half an hour, then you build up the running intervals and decrease the walking intervals.
I personally didn't follow any plan to the letter, but I did build up slowly and steadily adding a minute or two of running each time I ran. I was aiming for a 5k (Race for Life) so I was building up to the distance rather than a specific amount of time, once I could run for about 20 minutes solid I'd then walk it up to 5k, possibly with another shorter running spell at the end (I was on the treadmill so could measure distance and time easily). As I started running more of it (maybe running for 22 minutes with less walking before running again) I got quicker and quicker over the 5k which was a good way to measure my progress.
This time last year I was just about building up to being able to run 5k in about 36 minutes - now I can run it in 24, and I can also run half marathons (21k/13.1 miles) in under 2 hours. Anyone who can run now had to start somewhere. The lucky ones will have started running round while playing as kids and never lost the technique, the majority of people will have struggled with getting started themselves and give you nothing but encouragement. Stopping to walk is nothing to be ashamed of, it's a recognised training technique! You're working on building your endurance, and you can go for longer if you take walk breaks to recover between running intervals. If you can run 10 minutes twice with a break between you've done better than running 15 minutes then collapsing. This is why the second approach above is probably better than the first.
Two things you need.
PROPER RUNNING SHOES AND A SPORTS BRA. No excuses. Your feet and your boobs will thank you. It doesn't matter how stupid you think you will look, go to a proper running shop (ie not JJB etc) where they let you try the shoes on and actually *gasp* try running in them before buying them. If you explain you're just starting out they'll be gentle with you
One question I sometimes ask is why you want to run? Pinning down the reason can help your motivation. If you're aiming for a race, maybe a 5k, then think of how every run you go on will make that race easier and make you feel elated when you finish it. Think about the medal hanging round your neck at the end. If you're running because you've always wanted to run then you have it relatively easy. Think about how much you enjoy being out there and doing something you've always wanted to do, and how much better it will be once you can do it comfortably.
If you're running just because you've heard it's good exercise you've got the toughest job. Yes, you can think about the rewards of being thinner, but those aren't outcomes that you can control as easily. For a race or simply being able to run at all, putting in the training will get you there. If you're running to lose weight you have to accept that you'll have to keep running even if the scales don't move for a week or two to really get the benefits. If you start to enjoy running you'll keep doing it, but if you see it as a chore you might start to lose motivation sometimes. I'm not one to knock running, but for some people it is a step too far, they don't enjoy it and don't really want to do it. If that's you, by all means give it a try but don't be afraid to switch to doing exercise you prefer more, and maybe when you're fitter you'll feel more inclined to give running a good do. It's not impossible to move from this to running for the fun of it, but I think it's harder.
You'll get plenty of support on here, Kykaree and Frus are both doing their first 10k on Sunday so we all know how hard it can be to start running and keep working your way through those programmes.