You can freelance full time or in any spare time you have. Remember that most IT upgrades and change related work take place outside office hours and on weekends so there is no reason why you can't do it alongside your regular job.
I've freelanced for many years and here is what you need in order to get started – nothing.
That's right. You don't need to form a company. You don't need any qualifications. Heck, you don't need a fancy website or even a business card! I know it's hard to believe, but you can easily make a contact at a business event, tell them you can do X and Y for their network, and if they need either X or Y, they will agree to pay you Z dollars to come and do it. Don't tell me it can't be done because it is done all over the world every day.
When I started out, I put the word around that I could do Cisco networking. Small business owners started to call me to ask if I could help install routers, add access lists, VLANs etc. The best bit was there were no interviews and no boss. We agreed what needed doing, when it should be done, and for how much. I ended up getting paid a months salary for one days work. My eyes had been opened to the magic of freelancing.
If you want to do IT professionally then of course you will need to have a website, business cards, insurance and possibly a limited company or some sort of legal entity, but when starting out you can easily bill through your own name so long as you declare it for tax.
I just want to open your eyes to the possibility that with any knowledge you gain in the IT arena, you can easily charge customers to fix related issues and they will be happy to pay you handsomely for the work you do. In fact, I've never once had a client interview me or ask me for my qualifications or experience. They just seem to presume that if you are freelancing you know what you are doing.
If you want to build a successful IT freelancing business I do have a course on howtonetwork if you want to take a look. It's allocated free to all new joiners.