A pack is mostly a bag with shoulder straps, so it doesn't take a lot of learning in order to put one on and walk around. However, the more weight you put into that bag, the more challenging it is to remain comfortable as you walk around. That's what the hip belt, frame and load lifters, etc. are all about.

A day hiking pack is easier to use for the simple reason that you generally put less weight into one, so the design can be simpler and the consequences of getting a poorly made or poorly fitting bag are much smaller. I'd recommend just starting out by buying a day pack only and wait a while before considering a larger pack.

A critical piece of buying any pack is putting it on with weight in it. Ideally the weight isn't a sandbag, but what you'll actually carry in the bag, or something that resembles that load pretty closely. The best place to do this is at a local store.

Beware of sales people trying to sell you an expensive pack with lots of bells and whistles. A day hike is not a mountain climb. You'll mainly carry water, food, extra clothes and a few other items on a day hike. All that really matters is that this stuff fits inside the pack and when it is inside the weight sits commfortably. More expensive is not always better. You just need something durable enough the seams won't split on you. A few extra pockets can be nice, but aren't positively required.

All else being equal, a lighter pack is better than a heavier one. For day hikes you most likely won't need a framed pack, or even a hip belt. That only makes sense as your load approaches 20 to 25 lbs. A day hike load is usually closer to 10 lbs.

After you've put some day hikes together and found you enjoy them, you can start to think about backpacking in more depth and detail. The gear for backpacking will set you back more $$ than the gear for day hiking, so knowing you like to hike is a good idea before you sink much money into a hobby you may find uninteresting once you try it.

Happy hiking!