This is a common problem to new web sites. First, you check the ad code. If there is a problem, correct it if you can or contact google customer support. If there is no problem in ad code, the problem may be due to this reason. During the initial days of your web site, it won't get more views and therefore it won't get more ad impressions (less than optimum value). Hence, ad status will be shown as idle. I suggest you to promote your web site through your social networking profiles like face book, twitter etc., so that your web site starts getting some good number of views. Then, check the ad status and you will not find it as idle.
Hope this helps.