The main options for publishing a tabletop rpg are:
Get a publisher - get an existing company to do it. You have to negotiate with them on payment, who retains copyright of the game and so forth.
Pros: You don't have to worry about marketing, printing, layout or distribution!
Con: Less potential revenue for you. Less control over the game. Also, you want to market the game yourself anyway, to boost sales. The ideal situation is that you and the publisher work together to market the game.
Give it away online for free - get some webspace and do it. No money though.
Self publishing - you do everything yourself.
Pros: You control the game and keep the all the revenue yourself.
Cons: You have to do everything or get people to do it for you.
Normally self publishing is considered vanity publishing and looked down on in the publishing world. Self publishing in the RPG industry is not looked down on and many respected writers do it.
When self publishing, you have a few venues for publication. (also check out
http://www.indiepressrevolution.com/xcart/help.php?section=about )
PDFs - sites like rpgnow.com sell PDFs for a percentage of sales. Don't try to sell PDFs yourself, because setting up a secure merchant website is a pain and sites like rpgnow get a lot more traffic and paying customers than you will.
Offset printing - traditional commercial printing. You go to a printer and print a few hundred or thousand copies of your book. Depending on your options, this will cost a few bucks or more per book printed.
Print on Demand (POD) - go to a site like lulu or createspace and upload your book as a PDF. Then when a customer wants a physical copy of your book, they can order it and the POD company prints 1 copy and mails it to your customer.
Pros and cons
Offset gets you a much better percentage of the sales - a book might cost you 3 dollars to print but you can sell it for 20 bucks. In POD, you might only see 5 dollars of that sale. However, with offset, you have to invest upfront, store the books, worry about mailing and so on.
Some POD companies like Lulu let you offset print as well.
This is just barely scratching the surface of publishing.
Before you get into ANY of this, you need to answer a few questionsWhat is my goal with this game?
What do I have to do to MAKE it first? - writing, editing, layout, art, playtesting.
How serious am I in terms of investing my time, effort and money into it?