Go for somewhere you like, have visited before, or have knowledge of. Or else, select somewhere with the same language and similar trading methods.
Language is important. If you speak no foreign languages it's wise to trade with English speaking nations such as the USA, Canada, South Africa, Australia or Eire.
Your choice is far wider if you speak another language. If you speak Spanish, for example, you're ideally placed to deal with firms not only in Spain, but Spanish speaking countries in South America and elsewhere.
Having contacts in a certain country gives you an advantage. If you have relatives or friends in, say, Africa or Pakistan, this puts you in a strong position to trade with those nations.
Other practicalities that may influence your choice are the ease of travel, the strength of the currency and economy, and the (lack of) import and export regulations.