In every stage, you get a medal based off the score you receive in two ways:
1. Collecting as many Coins as possible by exploring the stages and finding them.
2. Get rewarded a few of them in the end of the stage based off how fast you completed it.
3. Completing Secondary Objectives
For Gold, you'll need 6000 Points or more. However, even if you get a Platinum, you can still get the achievement for the Gold.
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, but for the Golden! Achievement, you'll probably only get it after replaying the levels once or twice, or explore the stages a ton during your first playthrough.
If you're having trouble with the achievements popping, beat the Daycare Dash level as Buzz and it will force the achievements to pop (for whatever reason), 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.