i wanna start running how should i start ?

I started by using the 5k101 podcast that I found on itunes. It starts out really slowly, I think 30 sec running to a minute walking, but each week increases the running time and decreases the walking time. It really worked for me because, if I just tried going to exhaustion, then I always ended in failure, but this way I was walking because I was supposed to be walking and would start running again when I was supposed to. I did his 10k101 afterward, and then just kept running on my own. I did my first half marathon last month.

If you have a smartphone, there are several Couch to 5K (or C25K as it's known) apps which will walk (and run) you through it.

They will start off with mostly walking with intervals of short runs, then as you progress the length of the runs will increase and the walks will decrease.

There's even a zombie one called Zombies! Run! 5k, but that one costs a couple dollars.

If you've never ran before, do C25K. It'll help you get into it, just remember to run slow(able to maintain a spoken conversation.)

should i start buy walking for a few weeks or just start running

To a large extent it depends on your current fitness level, if you can comfortably walk for 30 minutes then you're reasonably placed to start running.

Broadly you've got two approaches, either use a structured plan with run/ walk intervals, or just run as far as you're comfortable, and add time or distance each session. Personally I used a Couch to 5K plan that got me to running for 30 minutes continuously in about 12 weeks. The plan is in principle 9 weeks long but I had a couple of diary issues that extended that.

For the other approach, if you can already run for say 10 minutes then start adding a minute at a time, then once you're at 20 minutes at 2 minutes at a time.

For me the former approach is a more reasonable session as it's going to take about 40 minutes and it'll cover more effort in a session, but for some people the walk breaks feel disruptive and they don't work.

Other things to think about:

Run every other day, give your body time to recover.
Build in some resistance training as well, to help avoid injury and in the longer term it'll help the running performance.
WEar decent runnig shoes, that are appropriate to your gait - I'd suggest going to a proper running shop for these as they can look at your gait and recommend a shoe that's appropriate.

Given your current weight it's a question of taking it easy, don't overdo it or you'll injure yourself. With that in mind I'd suggest a C25K plan, rather than just going it alone. There is a C25K support group on here as well that you can look into for motivation: http://www.myfitnesspal.com/forums/show/47-couch-to-5k-running-program-c25k-

The C25K apps are awesome!
I did that in the past and that was the main reason I was able to lose the 35 lbs I lost before. NOTHING got me sweating and burning calories like running, plus it is the ONE exercise I found I actually enjoyed (especially when I could run outside). This time around, I'm trying an app that is more geared to weight loss, but still starts 'from scratch' since I haven't run in well over a year. I do find this one to be tougher to get through, due to the incorporated sprints. So, if you are totally new to running, I'd stick with the C25K.
Good luck!

MeanderingMam is totally right. It depends on your current fitness level. A lot of starting 5k programs recommend that you are able to WALK for 30-45 minutes at a very brisk pace before starting to run.

Take it slow to let your body recover (frozen peas helped my sore feet a lot), stick with it when it's frustrating and have fun!

Edited to say WALK when I typed run.

My advise is to just walk daily for 30-45 mins. You will know when you want to start running.

I like the suggestions for C25K, however I will caution you that if you have zero running/jogging experience it can be too tough at first. Like someone else said, if you are currently able to walk (at an aerobic pace that gets your heart rate up) for 30-40 minutes straight without being dead afterwards then you could just start C25K with no problems.

Begin with the first day but if it's too hard then skip some of the 'jogging' intervals. And if you get an app you can look at the plan for each day so if you need to stay at one level for another week then do that. Slow and steady (and consistent!) wins the race when beginning running as there is the highest potential for injury if you do too much too fast.

Sometimes people think, "I'll just go run until I can't run anymore and add on from there" --- don't do this. Many times people can run pretty far if they have that mindset and it's waaaay too much the first day and then you end up hobbling around your house for the next 2 weeks unable to even sit down on the toilet to go to the bathroom comfortably, LOL.

Google C25K or other 'beginner running' programs (Runner's World has one too) and begin with those with the knowledge that you can modify it to your own pace....just as long as you are consistently running more/longer/faster over time

Ditto this. So much of running is psychological. I always ran as far as I felt like running on that particular day.

Sometimes people think, "I'll just go run until I can't run anymore and add on from there" --- don't do this.

fwiw I find this to be excessively cautious. While C25K is a solid programme, for many people it just doesn't work. Run/ Walk intervals can feel disruptive, and some people respond better to a single continuous run, extending it as the opportunity emerges.

Yes, there is an injury risk, but that applies to pretty much any plan.

Sometimes people think, "I'll just go run until I can't run anymore and add on from there" --- don't do this.

fwiw I find this to be excessively cautious. While C25K is a solid programme, for many people it just doesn't work. Run/ Walk intervals can feel disruptive, and some people respond better to a single continuous run, extending it as the opportunity emerges.

Yes, there is an injury risk, but that applies to pretty much any plan.

^ Good point. A full out sprint is going to be better for you in the long run

Given that a long run is in the order of 13 miles I'm not sure that would be achievable at 400 metre pace.