i don't know much about bikes but i need one for commuting. i went to Walmart and saw a mountain bike. i could not find a salesperson so a customer showed me about shifting gears. only thing was he did not tell me i needed to be pedaling when shifting gears. so at the store, the gears were really hard to shift and it even hurt my wrist. would shifting while pedaling have been much easier?

Yes, then you twist the shifter it pulls a cable which moves the rear derailleur. When not pedaling it will bind because the chain is not actually moving to another gear.

thank you!

Yes, the chain must be moving when you touch the gearchange controls.

Otherwise it won't actually change gear & you'll probably knock it out of adjustment if you forced it whilst stationary.

Get out there & ride around. Avoid big down hill runs until you got some chops.

Twist grip shifter? Cheap twist grip shifters are notorious for hard, poor operation and early replacement.
My advice - up your budget for a better bike without them

Last bike I had with twist grip shifters, the shifter didn't have much friction and wouldn't stay in place, so you had to hold it to stay in the gear you wanted. I was glad to get rid of that bike.

If you buy an internal gear hub bike, then you should be able to shift the gear even when the bike is stationary.

thank you everyone for all the advice!