First of all welcome to a fascinating aspect of collecting and realize it is a TOTALLY different animal than coin collecting.
This in mind my first suggestion is to buy a book or some books and do some reading. I am by no means an expert but believe me one of the first things I did when I started collecting was buy a book.
Here's the story in short. I worked in a truck stop years ago. People knew I collected coins there but my knowledge of paper money was ZERO.
One day the bookeeper called me and said a bunch of $20 notes from the 1950 series' had rolled in. I had $220 available to spend so I went down and chose what I thought were the 11 nicest. Can't lose at face value.
NEXT thing I did was go to the local shop (where I now work) and buy a cheap Blackbook. It gave me an idea of retail and how to grade. Those were my first two concerns.
I posted them on
ebay with big pictures and was getting $26-28 per note. Hey, I was onto something here so I started reading more, making contacts and always learning. I was always up on what was hot in small size and what was not.
In the interim I got into star notes. Was I excited when I found my first one but I soon found in a business that handles a lot of cash I often saw $100 face or more in star notes in a single day.
It was time to start really picking and choosing as my salary would certainly not pay for what I was finding.
You see at first it was all about what could I make money on to me. I had no real interest but a good source. Figured if I sold paper at a profit I could buy more coins. Problem is I got hooked on paper as well.
There was actually a few years where I barely touched a coin. ALL paper but now I do both.
I learned to discipline myself on star notes (not too well though as I still have about $2000 face stashed away). I found a few hundred web notes and well over 2000 Forth Worth 295 engraving errors.
Found some great errors, older notes, radars etc.
I read and I made contacts.I figure at one point in my truck stop days I was going through $3.6 million per year. Not even my money and all available at face value. The only thing I miss about that job actually.

So why am I rambling about this? Paper money can be a lot of fun and the pickings from circulation can be much greater than coins. HOWEVER, the out of pocket cost even at face is much higher.
This is why investing in a good book or two or several is a good idea.
Even then you have to be realistic. I'll keep a start note with a printing of 640,000 or less PERIOD. NOT one that has a total of 1.28 million but such and such was a short run.
Why? - because how many people are actually collecting those?
It's fine to keep those but realize the "value" is often well over what one can really expect.
I've seen some pretty knowledgable paper collectors here so feel free to chime in here folks.
Newbies - keep asking the questions and PLEASE do some reading and research. It's easier to make decisions if you're armed with your own information.
Clembo