The game has 93 on disc songs, meaning you have to add another 22 to get this achievement. The choice is yours on how you want to do it. You can import songs from previous Guitar Hero instalments (Guitar Hero: World Tour, Guitar Hero 5, Band Hero, Guitar Hero: Smash/Greatest Hits, Guitar Hero: Metallica), this will give you more than enough tracks to get the achievement (but you will have to pay for them). The second is to buy DLC from the GH Store, but this once again costs money with single tracks costing 160MSP or a track pack of 3 costing 440MSP (There are several Track Packs available for free from the marketplace, downloading these also count towards the achievement). The simple and free way to get this achievement is to either create your own or download songs from GHTunes for free. This achievement should unlock after you have gained the number of tracks and enterd the quickplay+ song selection screen.
Note: There is also 17 Neversoft created GHTracks on disc, this means you might only need to add another 5 to get the achievement.