You should obtain all necessary visas before you leave the United States, because you will not be able to obtain visas for some countries once you have departed. Apply directly to the embassy or nearest consulate of each country you plan to visit. Passport agencies can not help you obtain visas. You will need to give or send your passport to the official foreign embassy or consulate. You will also need to fill out a form, and you may need one or more photographs.
Many visas also require a fee.
The process may take several weeks for each visa, so plan accordingly and apply well in advance of your departure date.