So I saw someone ranting about how hard is to do a webcomic, very negative, and when I asked her about it I was given to understand that this was answering someone's question of "how do I become a professional webcomics creator?" I'm a hobbyist. This isn't my job, but I consider part of the hobby to be treating it like it could one day be my full time job, so here's the advice that I would give.
1. Do it because you love it. It should be a joy to work on. If you have this attitude whether you are able to make it your career or it's forever just a pastime for you, you will not regret it. Time spent on something you live is always time well spent.
2. Brad Guigar always says "Make sure you have consistent, significant updates." and I think that's some of the best advice one can give. Think about what kind of comic you want to do, how often you want to do it and what a significant update for that would be. You might post one gorgeously colored, dense page a week or be like me and post small, simple updates daily, but make sure you keep to your chosen schedule and that what you've given is satisfying for that time interval.
3. Create a buffer before you start. Having a backlog of strips means that on days I don't feel like working on the comic, I don't. It keeps me fresh. It means I'm always excited when I work on my strip and I don't have to stress about it.
4. Work large, when you shrink down your art it will look cleaner and smoother. Chose a trim size for your strips that would easily translate into a book. This will save you a whole lot of angst later. Save your files in an organized manner. Use light marketing services like ProjectWonderful to spread the word about your comic.
4. Then keep going and going and going. After a few years evaluate how much audience you have. If it looks like you've got enough base to monetize on, take advantage of all the great tools out there like Patreon and Kickstarter. Think about going to conventions and start making the call on if more time spent on the comic would equal more income for you and is it of more value (money or peace of mind) than your day job. And if you haven't gained that critical mass of audience, hey, that's OK. Like all creative industries, few will make it big, but if you've enjoyed the ride then it doesn't matter.