It's a very small (in size) complete computer!
You need the following:
a) power supply, which can be from a USB adapter
b) a micro-SD card
b) a monitor and a lead to connect it to the micro-HDMI socket on the Pi
c) a keyboard with a USB connector
d) a mouse with a USB connector
e) network connection - wifi is often fine but some models of Pi require an Ethernet fly-lead to connect to your Router.
f) wifi password - if using wifi
You may also need a second computer with the ability to put things on the micro-SD card, although sometimes that part has already been done.
The stages are:
i. put the software on the SD card and fit it to the Pi
ii. plug everything together
iii. switch on the power and follow the instructions on-screen
best source of information:
https://projects.raspberrypi.org/en/projects/raspberry-pi-setting-up
post any further questions here!