I want to know where can i write my website descriptions?
If you are talking about meta tags, you can include them in the headerinclude template for global tags.
In your 'headerinclude' add this:
<meta name="description" content="Description goes here"/>
exactly i need this but where i go Admiccp => Templates ==> then what next?
Don't put it in headerinclude - that's really bad for search engines. Using the same description on every page makes it seem like there's duplicate content. You'd be far better to put it before the </head> in the Index template. You can then use Google SEO to generate the descriptions for threads.