If you don't have another computer to test, its gonna be very hard to learn.
Is this for work or personal study? if work, make them give you a test bed. If at home, get a 2nd hdd and multi-boot. Having a test bed is the best way to learn.
Screwing around with AD on a live domain is like placing your knackers on a railway line during rush hour!
*raises hand* Got first hand experience of all those trials and tribulations.
I started my job as I.T. Manager 2 and a half years ago running a domain with 5 servers and approx. 50 clients. I've had no formal training at all. I scoured the net, spoke to others, and did trial and error testing to accumulate the knowledge i have now. I'm studying my A+ and MSCA/E right now, but its not even touched AD yet.
I probably know less than 50% of AD, but i manage to keep our system running ok.
To repeat; its hard without a testserver setup.