Decide what kind of retreat appeals to you. Meditation? Therapeutic? Pharmacological? Something else? Then think about how long you can logically, and easily, be away from home. And finally, figure out how much are you willing to spend on it.
With those parameters defined, research your options. Do some due diligence to get reviews or find other people of have been there. Again, retreats require trust, particularly if you are doing something like flying to Peru for an ayahuasca retreat where you will be expected to take mind-altering drugs.
There are retreats dedicated to specific forms of meditation, such as several days spent being guided through mindfulness practices. While the types of meditative retreats available are numerous and varied, these are three examples: