OK so you want to watch this video and learn how to manually do bed leveling. However if you really want to go over the whole machine (perhaps even tearing it down and rebuilding it so you know where everything is an can assure that it was assembled correctly) the you want to start with this video.

Once the machine is set up, it's time to install slicing software on your computer. Cura is a popular and free option, with support built in for the Ender 3 (you have to set up a new machine in the software and tell it what model you have). Following that, find something to print! There are many sites like thingiverse where you can download models from, but you want to start with something small to work out the bugs without wasting a lot of filament. Look for something like a calibration cube ("calicat" is my favorite) which will provide information on how well you have your machine set up.

Also to consider... if you live in a humid environment and you were given filament with the printer that is NOT in a sealed bag, you should be aware that filament absorbs moisture from the air and "wet" filament will print like hot garbage. Insanely cheap filament like the $12 stuff you find on Amazon can also cause problems, so stick with stuff more in the $20 range until you have some experience in working with your machine.