Clean the bike spotless. It's easier to detect wear and serious issues like structural cracks, etc. with a clean machine. It's easier to work on a clean bike...ask any mechanic about that. A clean bike is a bit more efficient. A clean bike can inspire.
As dabac stated, go to the Park Tool website and read, study and watch the videos. Buy the tools you need as you need them or buy a few in advance as your budget permits.
Watch the Art's Cyclery "Ask A Mechanic" series of videos.
Hit up YouTube with your specific issues...such as, "How to adjust shimaNO Ultegra rear derailleur", or whater issue you're facing. Odds are someone has already solved your problem and became an internet star by video recording the repair. You'll often find multiple videos on the same repair. Watch, listen and learn.
Lastly, as dabac again advised, ask specific question on the various forums. Every forum has experts on a limited number of subjects, general repair guys and mechanics, brand-specific people and it may take a bit to sort out which person is offering you the best advice, the correct advice or is just confusing you.
Start with cleaning (lots of YouTube videos on the subject), slowly work you way into the simple maintenance tasks and maybe see your local bike shop for in-person basic mechanic courses. Several of our area shops offer evening classes taught by mechanics on how to repair and adjust your bike.
Good luck! Wrenching may or may not be as much fun as riding, but it is certainly rewarding to do your own work and you'll learn a lot by doing it.
