This is pretty basic as far as working on cars goes. You're better off having someone beside you explaining what to do. It's hard to point over a computer.
First you remove the drain plug and the oil will run down your arm into the pan, (that's sarcasm, unless you're not careful). That pan should be large enough to hold two gallons. There's not that much oil going to come out but it's going to slosh around when you try to slide it out later. How far away the oil lands in that bucket will change as it slows down toward the end so you have keep moving the bucket or it has to be plenty wide.
Put the drain plug back in, then unscrew the oil filter. On some GM engines they conveniently put the oil filter inside a really big 3" diameter drain plug and you need a pliers to pull it out. You WILL have a mess running down your arm on that one, (no sarcasm). For the normal screw-on filters, you need an oil filter wrench to loosen it. The most common ones look like a metal band with a pivoting handle. On a lot of their engines, GM hides the filters where you can't see them and can barely touch them. Often you need some special tool to get them loose. Use your finger to place a light film of oil on the rubber ring on the new one to help it seal.
Before screwing the filter on, BE SURE THE OLD RUBBER RING HAS BEEN REMOVED FROM THE ENGINE! If you don't see it on the old filter or on the ground, reach up and peel it off where it stuck onto the engine. If it sticks on, that's called "double-gasketing", it will blow out when you start the engine and all the oil will be pumped onto the ground. Screw the filter on until it stops turning easily, then go about another 1/4 turn to be sure it won't leak.
Pour in the specified amount of new oil. Depending on the engine size, that's usually 4.5 or 5 quarts. Put the cap back on to prevent oil from spraying out, then run the engine for about five seconds to pump oil into the filter and fill it. Stop the engine, remove the dipstick and wipe off the oil that splashed up onto it, then use it to check the level. It should be between the "min" and "max" marks. It won't hurt if it's slightly too high. If it's a little low, wait a minute for the oil to drain down from the top of the engine, then check it again. Add a little if necessary. Wipe slobbered oil off the engine and car's frame, and clean up the mess on the ground. Haul the old oil to a place that accepts it for recycling.