For neck strain I'd say you were likely pulling on your neck/head, rather than stressing the core ab muscles to do the crunch work. You could try crossing your arms over your chest instead of the traditional "hands behind head" position. Lift from the shoulders and contract the "gut". Keep your back straight (shoulders and chin to the sky, not to your knees). Don't be flinging yourself up, make it a slower but more controlled movement. Yes, it is harder that way.
I have to chime in with a note about how important it is to strengthen the back muscles too. It's not uncommon for folks heck-bent on defined abs to neglect the back, which can lead to back pain! Don't focus on just one side of the body, my friends.
Also, highly-defined abs = tight muscles. The physical ideal is to be limber *and* strong. OK, so it's my physical ideal, heh. Don't neglect the stretching!
And jorgelito, I'd recommend hiring a personal trainer. A good one will show you what to do without depending on a gym, etc. It can be an expense, but you will save time (that most precious of all commodities), effort, possibly prevent injury, etc. Shouldn't take more than a few weeks/sessions to get started on the right track.