i want to place google ads in my new blog.and is their any rule to place ads after some particular posts?at present i have very few posts in my recent blog so is it good to place ads suggest .
No, go ahead and place it anywhere - you can get different type of ads - link units, actual ad units, you can place them anywhere you want
Post them anywhere you like, as often as you like. Just make sure you get your own content in there too.
The best would be if your blog is at blogspot, because they have the placement where you can specify where to place the blogs, also blogspot belongs to Google. You would be more certain that you wouldn't be flouting their rules.