It does not matter if you use a computer or draw the map by hand. Where, and how to start are questions everyone needs to answer. Additionally what type of map to create. Do you want to make a 3/4 perspective, a top-looking down, etc. These are questions that need to be answered. Also everyone might not answer them the same way. Actually I am sure that we all start differently. When I am starting a new map or fixing an existing map I start off with a blank piece of paper and a piece of collage rule line paper. I will sketch out the city, country, or whatever on the blank piece of paper and take notes on the lined piece of paper. This is not art or even will like a map. This allows me to figure out what type of map I want to do and what information i want on the map.
Now you are doing a map of a city. Here are some questions
What is the tech level of the city. Are you doing a medieval or future city, or what have you.
What buildings do you need or want in the city?
Does the city have a city wall?
Is it on or near water?
How big do you want or need the city to big?
This is a start. I hope this helps.