OK if the salesman couldn't give you any more advice than that, go to a different bike store. I might even go so far as to say a "real" bike store.

IN GENERAL: multi-speed bikes rely on the chain being moved or "derailed" onto a different gear that is a different size resulting in a different gear ratio.

The higher the number, the lower the gear ratio (purely made up examples: 1 = 1:3, 2 = 1:2, 3 = 1:1). This means the higher the number, the more effort it takes to move the bike at a given speed (and the slower the pedals move). The lower the number, the less effort (and the faster the pedals move at a given speed).

In "1", the amount of effort is SO small that you didn't even feel any resistance. Had you stopped the bike and started again you would have realized that there is resistance and the chain is still connected, but by the time you're moving at a reasonable speed you'd have to be pedalling like mad in order to be accelerating the bike any more.

With deraileur-style multi-speed bikes, you should be pedaling forward gently (not applying a whole lot of pressure to the pedals) while shifting. Obviously this can take a little getting used to, for example if you leave it in "8" when you stop it will be a LOT of work to get going again (and you can't shift into a lower gear until the pedals are turning).

There are two deraileurs, the front, and the rear. The right handle (8 gears) controls the rear one, the left handle (3 gears) controls the front. Technically this gives you 24 combinations (3 * 8 = 24) but in reality you probably shouldn't use 3 - 1 and 1 - 8 because they are on opposite ends of the rows of gears and wind up angling the chain rather sharply (resulting in increased wear long-term... won't hurt it if you do it like once or something).

He probably told you not to shift the left one because front deraileurs don't usually operate as quickly as rear ones (I think just because the front gears are usually larger, so it takes longer for the chain to move from one to another) so shifting the rear is quicker and easier for someone who has never used a multi-speed bike before.

The front gears are widely spaced and choosing a different one results in a different range of gear ratios... you wind up playing with them to find the combos you will use most. For example, my bike is also a 24-speed (3 front 8 rear gears). I usually use 2-1 to start, on up to 2-7 or so when moving at a moderate speed (10-12 mph maybe), and get into the 3rd front gear when going faster then stay in like 3-4 to 3-8 depending on how fast I can get going. I only go down into the first front gear when moving slowly over difficult terrain (gravel road or steep hills).