Here's a guide for unlocking all the Buddy Coins in the game, these buddy coins are required to unlock a playable character, each stage has 4 and when you get them all in a level, you get the ability to play as them in the level where you collected them. For example, if you got all 4 Buddy Coins on Fancy Drivin, you can play as McQueen in that level, but not the other two, you need to get 4 more in Bomb Squad and 4 mroe in Convoy Hunt for example. Afterwards, play the entire world as the characters and the achievement will unlock.
Warning: This game is pretty glitchy in terms of the achievements, most of the time, you'll only get 1 achievement after you beat a stage, even though a lot of times, you'll should have gotten multiple at once. So i don't recommend going for a lot of achievements until you get a good chunk of the easy ones.
If you're having trouble with the achievements popping, beat the Daycare Dash level as Buzz and it will force the achievements to pop, you'll need to unlock Buzz in Daycare Dash first, so just enjoy the game for a while until you get most (if not all) of the easy achievements, you might even get a Gold by accident.