The remake of the first game that just came out this past week is made to try and introduce more people to it. Its got a story line mode that's a tiny bit toned down at the start to introduce new players to the game concepts. So either that one, Etrian Odyssey Untold Millenium Girl or just get 4. Either way. But Untold Millenium Girl is probably a bit more approachable for a new player.
Its on the eshop or you can get it on amazon.
The other 3 older DS titles are also on amazon for around 20 ish dollars apiece.