Yes, obviously she is probably the most famous female pop musician of the 21st century.
But how come we know that?
Well, if there wouldnt have been Jay-Z, who brought her in that whole music business she would probably still be signing in her flat.
So first of all, have a contact in the scene - that helps you get in the press.
Be different, people dont want to see and hear what they have been seeing and hearing last year - htey want new styles!
Be crazy and make something that people associate you with. Get an image- Rihanna is famous for being the "Bad Girl", innocent font -but catchy, naughty color: 

And again RED in Rihanna's latest video:
Only Girl
http://www.youtube.com/watch?v=pa14VNsdSYM
She used the colour red -everything is red, even her hair:
She is not only on YouTube, MyVideo, MySpace, Facebook and Twitter, she has a own homepage and several fan pages where her latest songs and videos are shown.
But most of the people who dont know her yet, wouldnt browse for her, so what can she do to extend her number of fans?
So Rihanna needs to be there where potential audience is, e.g. on big music festivals, where other artist are performing and therefore different fans are. She needs to present on MTV and give interviews about her new video. She needs to market it to the audience by being present, as well by going to talk shows and talk about her past and future - to satisfy the audience thirst of news!
Freddy