Welcome to AA! Please make your way to the General Discussions or Getting Started sub-forums in the FW
section as you'll get a lot of good advice there.
But right off the bat, I can tell you that even a single goldfish is too much for a 10g. Even if they're not that large yet, they have a HUGE bioload, meaning they produce a lot of ammonia. I'd recommend to get a larger tank or rehome your goldie. You can get fish that stay small and aren't heavy ammonia producers. Something like a school of tetras. It sounds like you have more than one goldfish in there?
The key to crystal clear tanks is to not overstock, don't overfeed, have a good quality filter, and stay on top of partial water changes. I do mine once a week and change about 30% of the water. Also, to avoid algae, don't leave the lights on too long and don't setup a tank near a window.