What country you go to depends largely on what countries are currently requesting aid from the Peace Corps, what skills are needed in those countries, and what technical (work) skills you possess. Sometimes the language skills you have already developed are an additional factor.
Your Peace Corps recruiter may encourage you to state a “preferred region,” such as Central Europe, Africa, and so on. Stating that preference does not necessarily guarantee that you will be placed there — only that if your knowledge and skills fit the need of sites in that region, you will be considered for placement in that region. It has been said repeatedly by Peace Corps recruiters themselves that the more flexible you are in your expectations of where you will be sent, the more favorably your stated preferences will be considered. However, if there are specific countries or regions to which you know you would not accept an invitation, it may be wise to let your recruiter know that.