Upload your work to file hosting services like YouTube and zSHARE. This will allow you to send links to your music to websites that don't accept file attachments.
Write a short biography of your history as an artist, what and who inspires you and the direction you see your music taking. Some sites will be more likely to feature you if you do some of their work for them and provide them with some decent copy. Don't be afraid of using a little poetic license to get yourself noticed.
Employ the services of a professional photographer to take some promotional shots of you. Again, websites are looking for quality content. Sending in some small photo files you've taken on your phone's camera isn't going to impress.
Set up a website and social media presence. Online publishers will be keen to see that you have some momentum behind you and how your fanbase reacts to your work.
Establish a street team to create a buzz about your music at a grassroots level. Give some of your fans the opportunity to earn free passes to your shows and complimentary merchandise in exchange for handing out fliers, putting up posters and generally spreading the word about your music. Websites will be keener to feature you if they know you're being talked about.
Send your music, biography and promotional photos to hip hop websites directly. Some, like Hip Hop Linguistics and World Star Hip Hop, actively solicit submissions from aspiring artists. With others, you'll need to get their contact details, send your press pack by email or post and follow up with a phone call.
Employ the services of a PR company. Outsource the job raising your online profile to the professionals. A music PR company that specializes in hip hop should have contacts at a number of leading hip hop websites and should have little trouble getting you the exposure you want.