This is a relativity easy achievement that will either be a) slightly time consuming or b) the first achievement you earn as soon as you boot the game.
The EA SPORTS Football Club is tied to your Gamertag and carries across different versions of FIFA, so if you've played 12 or 13 for example, your level carries over. Or even FIFA14 on the 360.
If you aren't already level 15, all you have to do is reach it. This is quite simple, and XP is earnt from playing games, completing matches of the week, playing online, playing FUT. Pretty much everything will earn you XP, with difficulty scaling the XP payouts.
You can check your current level in the top right of the main menu. You'll see a bar that says something like (Example) 4 250/1000. This means you are currently level 4 (11 to go) and have earnt 250 XP towards the 1000XP required to hit level 5.