Depends, on what you are looking to get out of the series and what systems you own. As there are 3 games that could fit your needs: Fire Emblem: Blazing Sword (just called Fire Emblem outside of Japan, as it was the first FE game to see an international release), Fire Emblem: Path of Radiance, and Fire Emblem: Awakening.

Blazing Sword is perhaps the best to go for if you are looking to get a good understanding of the fundamentals of the series and want a good jumping off point for all of the other games in the series. As everything introduced, mechanics-wise, is present in nearly every other FE game. So a great entry point, if you think you might be interested in playing all the FE games. It was originally released for the GBA. But if you have a Wii U, it's available on the eShop.

Path of Radiance is also another good on basic fundamentals (though, there are a number of additional mechanics not found outside it). And if you are looking for a little bit of s challenge, it can be more difficult thsn Blazing Sword. The only issue with this game, is that it is difficult to get your hands on. For one it was only ever released on the Gamecube. Meaning you would need either a Gamecube or Gamecube-compatible Wii. Plus, it can be rather expensive.

The last one is Awakening. Awakening is perhaps the most newcomer friendly of the 3. With the ability to disable permadeath through casual mode and the ability to grind for more experience and reclass, if you feel you are struggling make it very accessible for those who might be intimidated by some of FE's mechanics. And being on the 3DS, it is the easiest to get ahold of out of the 3. The issue with it being that it introduced a number of new mechanics that weren't in prior FE games as well as reintroduced a number that hadn't been seen since before the series left Japan. So it might make it more difficult to play some of the older FE games, due to the absence of those mechanics.