My outdoor faucet leaks and the handle is loose....

You have two things going on here. Fix the handle first, there is a screw in the middle of the handle, tightening that should remedy the loose handle. Now there is a hexagon nut below the handle, sometimes just tightening this nut will correct the leak. If tightening the nut does not correct the leak, then you need to replace the cartridge, which is inside under the nut. If you have to replace the cartridge, then Yes, you will have to shut the water off at the shut-off valve in the basement. Hope this helps.

If it is just the handle no. If it is the entire fixture that connects to the pipe that needs replacing, yes the water will have to be turned off.

If the handle is one of those octagonal ones that has a screw in the middle, as long as you turn the spigot off tight, I think you wouldn't need to turn the main water supply off.

Yes

leaking problem; under the hex nut there's a washer ( not easy to see, it's embedded in the nut & it's black) that as you tighten the nut, the washer expands around the shaft sealing any water from flowing along the shaft and causing the leak.

Without shutting off the water, first you tighten the hex nut, but only enough to stop the leak, because you may need to tighten it again in the future or you may not. If that doesn't stop the leak, go to Home Depot and talk to someone in plumbing and he'll give a product that's used to correct shaft leaking problems. It looks like white rope, it contains silicone, very inexpensive. Loosen the hex nut and wrap one turn of this rope around the shaft, replace the nut and tighten enough to stop the leak. Because you've added additional material on the shaft it may a little hard to get the nut back on to catch a thread, but it's doable, put some pressure on the nut when trying to thread it back.

Thanks!