Just start.
Seriously. Don't "wait until you get better", don't "wait until your genre is popular." If you want to start a webcomic, then just start. Draw some pages, upload them, take part in the community, and the readers will come. The only real piece of advice I have regarding that is to regularly update. Choose a day to update that works for YOU (whether it's once a week, three times a week, etc.) and stay consistent to it. Don't pick something that's too much work for you, but pick something that works for your comic as well (ex. if it's a gag comic, those work better being once a week/multiple times a week; only story comics can really get away with being updated once a month/once every two weeks, etc. because of their content).
And have fun with it.
Most people's answers: "It should be of decent quality, go to others for advice and such."
My answer: No.
People come for the art, but stay for the story. If your art is absolutely gorgeous, that's a bonus. But if you can succeed in telling your story with your images without confusing anyone, and if your content is good enough, then people will read it. People are often drawn to comics faster if you have great art, but again, that's just a bonus. You don't have to go far to find comics that successful but have sub-par artwork (XKCD, Cyanide and Happiness, Poorly Drawn Lines, etc.)