how to use seo in website?

I am not clear what do you want to know? Did you built your website? If yes, then complete your on page as a first step of SEO. Selecte Title, Meta-Tag, Keywords selection, Description and then start off page.

I think you are talking about "on page" aspect of seo. For this you can check for on page factors. Main factors to see at first include:
+ META Data (Title, Description)
+ Content (seo copywriting)
+ Keywords usage throughout website
+ IA (Information Architecture)
+ Call to Actions
+ Landing Pages
+ Headings, Other SEO tags
+ Sitemaps, robots file
+ Duplication
+ Canoncalization
The list goes on!

You're right @Stacey_1. But which I have written is most important.

Couldn't actaully understand what you are trying to say over here by telling SEO in website !! I would say you need to be a little more clear or elaborate what you are trying to ask over here !!

It's quite difficult to understand as the question is not elaborated. However, For use of SEO and it's latest trends,I suggest to refer websites like searchengineland and searchenginewatch.