You can specialize in areas such as Architecting , Developing and Operations having to levels of each area i.e Associate & Professional.
Each areas has one test for all the levels you will be looking to complete. Where all associates will cost you $150 no matter where are you taking exams from.
& professional exams are $300 for each exams. But in-order to pass professional exam you have to be an associate certified.
Validity : Each exam (Associate & Professional) is valid for 2 years..!! Yes, the exams are valid only for 2 years and you have to get re-certified after every two years.
Here you can register for an exam: https://www.webassessor.com/
Practice exams : Before going for the actual exam you can also register for practice exam on same exam registration site. This is totally optional part. You only need to pay for practice exam once as the questions are the same each time for one particular exam.
Cost for any associate level exam is USD $20 and for professional level it is USD $40
You might want to go through some links(Exam Blueprints) even before you start planing for AWS Certification exams. You can get exam blueprint, syllabus, topic wise weight & sample questions