The Raspberry Pi computer unit, a small $5-$35 Linux computer, keeps its original cost low by not including three main things: a keyboard, a mouse, and a power cord. Here's a mostly complete list of what you'll need all together:
Recommended Base Unit, Case, and Book
Power cable - Your initial power cable should be 2A or more. I used to recommend using an old Android micro-USB charge cable, but many of those don't supply enough power. (The kits for the Raspberry Pi 3 have an appropriate cable.)
Keyboard and Mouse - If you have an old keyboard and mouse laying around, and it's got a USB connector, you can plug it into the Pi. If you don't have one yet, you can often find a nice combo set for under $25.
Storage - The Raspberry Pi 3 does not come with a regular hard drive like a laptop would, so you'll need to pick up a microSD memory card. The prices have come down so much, I recommend starting with a 32GB Class 10 or UHS-1 microSD card. If you're planning to try other OSes than just Raspbian/Pixel, you can use an old microSD card as long as it's at least 8GB and class 6.
Getting on the Internet: Wired or Wi-Fi If you buy the $35 Raspberry Pi 3, which is my sincere recommendation, you get an included Ethernet port, which is great, because you won't use up the USB port that's on the unit. Wifi and Bluetooth now come built-in on the Pi 3.
Screen/Monitor/TV - Your new Raspberry Pi unit connects to
either an HDMI screen or TV. You'll need an HDMI cable or TRRS cable (it lets you break out to the yellow/white/red jacks on an
older TV.)
If you are using a standard HDMI screen, all you need is a regular
HDMI cable. They're a lot cheaper online than buying in a store, especially
if you buy a combo pack. If you're using a computer screen, it might
only have a DVI port, and not an HDMI port. Here's a picture of what
the DVI port looks like. If your computer has this, you'll need an HDMI
to DVI cord, which is really cheap here at Amazon. I use this one
myself with an old screen that was collecting dust.
If you really want a new screen for your Raspberry Pi, I can't really
suggest you spend $2000 on a really big high-def TV.
You should be fine a regular decent quality LCD or LED screen,
which range in price from about ( $100 to $200 ). Here's a few that
should work well for you:
Carrying Case If you're going to be transporting your Pi unit, you might want to make sure the unit itself is in a cushioned case. Your typical digital camera case shoud work fine, especially if it has room for spare SD cards.
Once you get your unit hooked up, which is really quick, you can use some of the following guides to get everything installed.
If you want to turn your Raspberry Pi into a Lego supercomputer, here is a link and YouTube video.
I'm not recommending you use a Raspberry Pi Zero as a desktop machine. It's better suited for projects unless you're on a super tight budget.
If you're just building a regular PC, here's a list of the parts you'll need for building that PC.