Somethings depend on what you actually want to do with the Pi. Just general purpose things like internet, email, maybe some LibreOffice things? As a media center with something like OpenElec?
There are a few basic things that are a requirement, not a wish list:
- a GOOD power supply. If you are getting/have the official one you should be fine. Lower cost and/or lower output supplies will cause instability - and things like everything just looking like it drifts off to nowhere or constant rebooting, failure, reboot, well you get the idea.
- you cannot put too much of a load on the Pi's USB ports. You would be much better off getting an A/C powered USB hub for much more than just basic things.
- as already mentioned a GOOD video cable. If it all possible connect using HDMI to a HDMI enabled device.
- a keyboard is a *MUST* when you first boot (although Jessie on my latest Pi 3 seemed to go straight to the GUI). Similarly a mouse is pretty much a must.
- as weird as it sounds, the size of the SD/MicroSD card doesn't have to be real large. If you plan on adding a lot of extra software packages or having a large amount of data you may want something a little larger, but an 8 or 16 gb card really is way more than enough to install the OS and run things.
Other things to consider:
- with the powered USB hub you can connect other devices via USB, such as the flash drives mentioned in an earlier post, but also external hard disks. I currently have a self-powered 4tb, a self-powered 1tb and a USB powered 1tb drive connected via a powered USB hub. This also allows putting everything but the bare minimum to load the OS on a hard disk - much faster. You still have to have a SD/MicroSD card to actually load the image, but everything else can be on the hard disk. There is a free script on Adafruit's site you can download that makes this extremely simple for the newbie or casual user.
- if it's not a Pi 3 you will need a wireless adapter if you want to connect to a wireless network where you live.
- you don't have access to a wireless network but do have either a local router or perhaps full-house wired networking you will also need a network cable
On a personal note it's just the little bit of stuff I use, I have a Pi 2 model B that I run as a server with a total of 6 tb of external hard disk connected via a powered USB hub and a direct connect to a router port, a Pi 2 I use as a media center using OpenELEC and getting all the media files from the server. That Pi is also connected directly to the router so the speed would be best for accessing the media files. I have a Pi 2 model B running OpenELEC in another room that is connected wirelessly to the server to access the media files. I have various other Pi 2 model B's and a couple of Pi 3 model B's that I am currently just experimenting with. One of the Pi 3's is connect to an external self-powered disk drive so I can try to test its performance as a simple general usage PC. So far I'm quite impressed that it handles the internet well (I use Iceweasle - a Firefox like browser for Raspian), does LibreOffice tasks great so far and for simple games like some of the card games it works quite well also. I'm not a gamer, so I don't know how it would for such things. As just a desktop PC for someone like my elderly parents who get online to check the net, look at news stories, use Facebook, etc., use LibreOffice Writer and Calc and play mainly card games it could be just the ticket for them in place of their laptop. I'm experimenting a little with one of the older Pi's to use a cheap censor to detect when my cat enters the litter box, sets a timer for roughly 2 minutes, then trips a relay to release air freshener (silly simple project but fun!).
To summarize, there are some basic things you really do need just to get things set up. After that it just depends on what you want to do with the Pi. After initial setup the Pi can run headless - no keyboard, mouse or display - and can be accessed via SSH client, such as ExtraPutty, from another PC. I manage my server that way and also have a few extra SMB shares set up so that the various PC's in the house can back up their data, retrieve or restore data, directly from the server. There is 1 Linux box in the mix, the rest are all Windows PC's that are running Windows 10 now. The shares for backups, etc, can be accessed directly from file manager in Windows. This is also how I move new media files to the server - just using file manager.
Don't be overwhelmed and don't hurry. Don' worry about making a few mistakes along the way and needing to reload the OS raw image to the SD/MicroSD card and starting over. If what you have access to right now (I understand - I'm on disability now) is a 32gb SD/MicroSD card (which also depends on the model of Pi you are using) then just go ahead and use it. Make sure the 1st thing you do after installation is go to the menu and run the raspberry pi configuration tool. You will want to expand the file system first - otherwise it won't see that 32gb anyway. Then you can set localization things - country, time zone, etc., set up for allowing SSH access, etc..
Don't be afraid to ask questions. These things are very simple once you've done them, but just about everyone has their own versions of "dumb questions"
Hope things go great for you!