Try doing as much free lancing in your spare time while also maintaining another job with stability. It takes years of successful
dedication to build up a client and work flow situation that yields the money you need/want (to live on/plus more) and in getting money at regular and expected periods (cash flows).
Instead of an all-in approach, I recommend doing it gradually (if feasible) given all this advice above. Get some clients, get some work. Keep your full time job. As work picks up... drop to a part time job. As work continues to pick up even more, you can relinquish your "job" completely. If you don't follow this advice or consider it deeply, I suspect a year forward from now you'll be looking back realizing that it is a reasonable strategy.
Aside from those issues, there is a lot more. To start a business, you really need to be prepared to give it your 100% (if you want to be successful). When you run your own business, there is NO SUCH THING as "hours"... you don't go to work or get off work at set times. You don't get days off or vacations or sick days. It is a serious time commitment. Typically, most sole proprietors work 60+ hours a week minimum for the first few years of business.
As rocknbil notes... also be prepared for all the additional roles you'll take on. Running your own hosting/design/development/etc company (starting off) means the whole game is on you. You won't just be writing CSS anymore or making cool graphics in Photoshop. You'll be doing book keeping, marketing/promotion of your own business, managing/dealing with clients & potentially employees later on, and a whole lot more.
If you're very serious about putting your all towards it, I say go for it! But don't run into it blindly. Do your research (as you are by posting here) to ensure you can make sound conclusions and judgments.
I also recommend before doing anything, that you do some research on business plans
. The below link should be useful (I'm hoping a Wikipedia link doesn't violate forum rules). Though typically useful for real business ventures in acquiring capital... they can also be very useful for people starting a business. It forces you to fully consider, formulate, develop, and commit to your business. (if done well/right)