bike saddle/seat at the correct height for you

If you call it a saddle, chances are you're probably good. Most folks, though, are riding around on bikes with the seat too low, and it's hell on the knees. People who are Really Into Cycling® will have a wealth of resources and opinions, but it's often ignored for people starting out, and that's a shame, as it's such an easy fix for long term health and happiness. For general purpose and long distance riding, at furthest downstroke your leg should be almost nearly fully extended with the tiniest bit of bend. You shouldn't feel like you are reaching far with your toes, and your knees shouldn't lock out, but should have a very subtle gentle bend. If your knees accordion outwards it's definitely too low. If you are unsure, try raising your seat slightly and see how it feels. It's often easier to identify when it's just too high than just too low, so keep raising then back it off.

There's a whole science to bike fitting that becomes more relevant the longer you spend on a bike, but everyone deserves a comfortable and efficient ride! Help your friends! Help your parents, especially if it's been a while for them.

One time you might want to lower it, if just for the short term, is for a nervous and newer rider. The sense of confidence from being able to securely touch the ground will outweigh optimal fit.

The video linked shows how to adjust your saddle height and angle. Many great guides abound online, just search correct saddle height.