You can't travel until level 15, which is why reaching level 15 is important. Once level 15, it's important to rent a Private Island with an airstrip. That allows faster, cheaper travel than otherwise, and you can carry an additional 10 items each trip, which means if you travel a lot, the Private Island pays for itself.
To actually travel, once level 15, you click on City, Travel Agency. (It's in the West Side section.)
Save this link. Bookmark it. It helps a lot when you're traveling for flowers and plushies.
http://travelrun.torncentral.com/Using it, you can estimate reasonably well to make sure you don't travel some place that will be out of stock on flowers and plushies.
Any of the flowers or plushies you travel for will have decent markup on resale. Most other items don't. If you can't get flowers or plushies, the next best option is usually some form of grenade (temp weapons), but as a rule, they don't make as much profit as flowers and plushies.
Xanax is profitable, if you get it from South Africa, but anyplace else that sells it will be higher than item market. But it's a long travel trip, and essentially costs you energy to make the trip. Plus it takes a lot of on-hand cash to buy xanax, which means if you get mugged as soon as you land, you lose a lot of money. Overall, for lower level people, I don't recommend running xanax.