Some good advice here and I think speaking from experience I'd have to say you have to ask yourself what you're happy with achieving (or not achieving) Making music, selling music, marketing music and the background stuff like videos/accounting/entourage/props bla bla all costs thousands even just to do one quite simple song/video. Unfortunately what some have said here is very true.. doing it yourself is one way to give it a go but to grown even a moderate true fan base you need first of all to get very well 'known'.. have some brilliant catchy songs under your belt... heaps of time and good equipment to perform them, and lastly a big dose of luck! The sad fact is, some things take off and some things do not, even if they are good.
If you're extremely confident about your music, then there is no harm sending off a few demos to labels (what have you got to lose?) Plus there is no reason why you can't go for an online presence AND send those demos off. Don't cut corners.. go for it if you really want it badly enough. Just remind yourself that there is a chance it may never happen.. at least to the capacity you might dream about. If you're happy with taking the journey anyway I'd never discourage people, because it's a great experience and along the way you make some great contacts too.
Lastly, I'll say that there are various ways to sell music.. Online. Offline.. from a bucket at your local gig house.
I started selling my first album 5 years ago and it took off remarkably well (mostly in my area because everyone knew me as a live performer singer, thats where I began with a dose of luck). Due to my popularity at the time I set up an online label and signed with a distro corp, and now I've got itunes and all those other online places managing my tracks. I'm happy with that. I took a long break from music the last two years, (to my detriment) but life took over. However, the original fans I created through my live gigs over the years are ever faithful. Some Ive never met but they get my fanmail and download my songs and I'd rather have that moderate little niche of supporters than takeover the world! So look at what you want ultimately.. if its the stuff legends are made of, you might have to push yourself completely out of your comfort zone for a long long time! If you're happy with moderate achievements or even small ones then take it one step at a time at your own pace.
Hope that helps somewhat.