Develop your idea. Every webcomic starts with a premise, and yours is no exception. Whether it's situational comedy or a years-long drama, set yourself up with an idea that will provide you with material for the long road ahead.
Set up your work flow. If you are drawing your comics by hand, set up your scanner and try scanning some images to determine the best settings to use. If you will draw your comics directly on the PC, calibrate your drawing tablet and practice with it until you are comfortable and accurate with it.
Create a few test strips. Your art is the most important part of your webcomic, and developing your style is an important part of your comic. Practice drawing your characters and write a few test comic strips to get a feel for your comic and the process of creating episodes.
Get a webcomic host. Either you can host your own comic, or you can use a dedicated webcomic host to do it for you. Dedicated hosts are great for new webcomics as they are free and they already have a system set up for archiving and navigating through your comics.
Set a posting schedule, and stick to it. Comic readers want to know when the next edition of your webcomic will arrive, and if you don't stick to this schedule, you're likely to be abandoned for greener pastures. If you can't do daily strips, semiweekly or weekly strips are a better choice than constantly posting late.
Advertise your comic. Many webcomic directories exist to help you advertise your comic for free, so start by registering your comic there. If you want to draw more readers, try advertising on a similar comic that is more popular than yours. Some of the visitors will stay and you can start building your traffic.