Need help getting the current page id from the url

[eluser]Unknown[/eluser]
So what iam trying to do is make it to were i click on my category i want to go into. on that page there are titles from that category. now iam making it to were i can create new topics inside the category but what i need help with is inserting that topic into the septic category_id in the data base that i would get from the URL. i really cant find out how to do this what so ever.

if some one could post maby a sample code or any help possible would be great if you guys need any thing from me to post on here to better explain this let me know.