This is a rather large question. Before you can choose or prioritize your target markets, you must make sure that you have a very clear and detailed vision and strategy. Has your product been designed with a particular consumer in mind? Is it a product that needs some kind of learning curve for consumers or will they purchase it just by having it available?
Once you know what your company's particular strategy is for this product, then you can begin the next step which is to research your available markets. This research is very important and will attempt to quantify the potential customers and sales in each of the markets under consideration. For example, if you are marketing a bubble gum product that is specificlaly designed for teens with braces, you will need to determine how many teens are in a target market, how many teens have braces and how many people chew gum. You would not choose to enter a target market, such as a third world country, that neither has a significant number of teens with braces nor a habit of individuals chewing gum.
Once you research your various target markets, you can prioritze them according to potential sales. Once you have done this, you will have to look at obstacles and costs for expansion. Would you need a factory there? Is that possible? Does the country/market require local ownership, etc.
When you have all of the research done about the potential sales and the potential costs of expanding. You can prioritize which target markets make the most sense based on research.