Because you want it done right.

When anyone mentions DIY they tend to put themselves into one of two camps.

Camp One assumes it's something they should easily be able to handle. 
Nothing to it.

Camp Two is too often intimidated by the entire process. 
As in here's my checkbook.

That's too bad because in many cases-more than you think-you can and frequently should DIY projects.
Not everything of course, but probably more than you realize.

My mind quickly goes back to every project I've ever had to fix when someone who "wanted it done right" didn't.
 Often badly. 
Leaving me to figure out not only why something isn't working, but now having to undo someone else's work.
On top of everything, hiring a "professional" doesn't mean anything ever gets fixed anymore. 
All you can count on is the bill.

So where should you begin?
The key is always preparation.
 Always.
The next time you find yourself faced with a DIY project, particularly one you might have previously considered hiring out, consider reconsidering.
Instead commit yourself to fixing it yourself. 
Assuming of course you know you can do it safely.

Start by writing down everything you know your going to do and what you'll need to do it (tools, parts,etc).
This is also the time to prepare the project as best you can. 
This includes cleaning the area where the work takes place or any other annoying little chores that could wind up derailing you.
DON'T just start pulling things apart expecting to just figure it out as you go along.
When possible do as much research ahead of time to familiarize yourself with any issues you might encounter.
In time and with experience most projects will become easier. 
Most importantly you'll learn how to go about fixing things in the first place.
 Different jobs will require different approaches. 
Soon it will become familiar to you.

Get some practice in by tackling some of the smaller household projects you've been ignoring.
This will, if nothing else, put you in the right mindset and reacquaint you with your dusty toolbox.