B737NG flight crew entered an active taxiway without clearance while attempting to taxi to what they believed to be their ramp control directed 'hold spot' on the ramp at DTW.

Narrative

I was unfamiliar with DTW. We were not planning on a southwest taxi. I was thinking (and briefed) that we would taxi northeast. This was the short route to runway 22L from our location on the ramp. The commercial airport chart is a little confusing; because there was no clear indication were the ramp ends and the active taxiway starts. There is also no clear signage at the airport letting you know where the ramp ends and the taxiway begins. We pushed off the gate. We thought the northwest ramp controller told us to taxi to Spot 1 and contact ground. We began taxiing southwest bound. I looked down at the commercial airport page and did not see Spot 1. I asked the First Officer where it was and he said it looks like around the corner. This made sense because we were taxiing over the spot numbers in order; counting down. As we approached Taxiway K; we heard ground on COMM 1 tell us that we were taxiing on an active taxiway. I stopped the aircraft. Ground gave us further taxi instructions to the runway. We departed without further incident. I could be more vigilant in taxiing in unfamiliar areas. I think that the commercial airport chart could be improved to make the ramp area clearer. I was unfamiliar with DTW. We were not planning on a southwest taxi. I was thinking (and briefed) that we would taxi northeast. This was the short route to Runway 22L from our location on the ramp. The commercial airport chart is a little confusing; because there was no clear indication were the ramp ends and the active taxiway starts. There is also no clear signage at the airport letting you know where the ramp ends and the taxiway begins. We pushed off the gate. We thought the Ramp Controller told us to taxi to Spot 1 and contact ground. We began taxiing southwest bound. I looked down at the commercial airport page and did not see Spot 1. I asked the First Officer where it was and he said it looks like around the corner. This made sense because we were taxiing over the spot numbers in order; counting down. As we approached Taxiway K; we heard ground on COMM 1 tell us that we were taxiing on an active taxiway. I stopped the aircraft. Ground gave us further taxi instructions to the runway. We departed without further incident. I could be more vigilant in taxiing in unfamiliar areas. I think that the commercial airport chart could be improved to make the ramp area clearer.