Hi, and welcome! If you haven't already, do read the site's FAQ and the "How to BookCross" info - both can be found under the "information" tab. But the basic answer about what to do if you find a BookCrossing book is to enter its BCID in the big box at the top of the page - or use the "journal entry" link under the "home" tab. That will let you enter a note on where you found the book, what you thought of it if you read it, or anything else you want to say - and if you make the entry while you're logged in to your BookCrossing account, the book will be added to your BC bookshelf so you'll be part of its history.
If you're wondering how to find a BC book in the first place, that's a different question. Browse the forum for threads on finding books or hunting for books and you may get some ideas!