Basically you can copy the solution for this achievement from previous FIFA titles. I've modified the solution - slightly - below to account for changes within FIFA 15:
Most FIFA 15 players will get this just by starting the game. (The current TA ratio for this achievement should prove that assertion.)
If this is your first FIFA game, or you somehow have played a lot of FIFA but haven't gotten much XP, well you get XP for doing various things in the game.
Examples include: starting the game, creating a pro, changing the name of your pro, having your favorite team get promoted in the EAS FC, opening FUT packs, etc. - but mostly you will get XP by playing and completing matches - either against the CPU, online, or in FUT matches of various flavors.
You'll get the first few levels out of the way fairly quickly. If you play FIFA with any regularity it shouldn't take you long to get to Level 15. And chances are, if you are reading this you are going to try to get as many FIFA 15 achievements as possible, and in the course of doing that you'll get to Level 15.
Your current level is displayed in the top right hand corner of the main menu (and on most every page you navigate through). Your current level is the first number listed in that box (and was yellow on my display). The second set of numbers (these were white on my display) indicates your current amount of XP, followed by a slash, followed by the total XP to reach the next level. This is a slight differentiation in how the numbers used to be displayed.
For example, if the numbers listed are as follows:
77 644,078/644,400 126,586
this indicates you are at Level 77, and you have 644,078 of the 644,400 XP required to get to Level 78. The third number is the current amount of FCC (Football Club Credits) you have to spend in the EAS FC Catalogue,
FIFA 15 limits the amount of XP (and FCC) you can gain in one "day" (be aware that FIFA 15's day is based on London time). You can check your current day's XP by hitting the start button
, and then going to the "My Activities" screen (also called the "Share" screen), which is the yellow square with the curved arrow coming out of it (you can move to it using the
or
buttons). Your current XP and FCC gained for the day - and the limit - will be displayed in an alternating fashion to the right of the "My Activities" menu title.
You are usually limited to 2000 XP per day, so if you are just starting out it will take you a few weeks minimum to get this achievement.
I can't verify this yet, but if FIFA 15 is similar to previous editions then you can increase your daily cap on those days you complete one or more Match Day Challenge's (from the Highlights of the Week screen). How much you increase your cap that day depends on what level you complete the challenge at. It's not much, but over 15 levels it can add up pretty quick.
Good luck!!