When should I change my gears?

You do not have to zoom down a slope when descending if you put more resistance on the pedals by using the "higher" or "harder" gears.  The lower gears are easy gears are for ascending hills.

Shift down to a lower gear = Ride uphill = ride on lower gears to climb uphill with less effort.

You are cycling on low gear when

1) your chain is on cog of smallest chainring = gear 1
2) your chain is on cog of biggest sprocket at the rear = gear 1

Shift up to a higher gear = Ride downhill = add resistance on the pedals and to reduce cadence (in your own term, heaviest).

You are cycling on highest gear of your bike when
1) your chain is on cog of largest chainring at the front = gear 3
2) chain on cog of smallest sprocket = gear 8 = you are running on 24 speed

This diagram of cogs on a chainring and freewheel of a 24 speed bike gives you an idea on which cog your chain is sitting on when you change your gears.

What this video if you would like a clearer picture of how your chain moves on the chainrings and cogs when you change your gears.