City campaigns are tricky, especially for a first timer, as you can't have everything in the city pre-designed so the PCs are bound to catch you out sooner or later by asking for something you've not thought of - shop, contact, inn name, etc. Be sure to have some reference material ready that can be dropped in any time, e.g.
- list of random names for people, inns etc
- list of descriptive words to cover the names so that you can make each inn, person, etc a little bit more memorable
- make some geomorphs of city streets & areas that can be used for anywhere in the city & dropped in for brawls, random encounters, etc
- similarly acquire some floorplans for inns, shops, temples & other buildings that you can keep in reserve and produce when needed - don't use them unless you need to!
- have notes on how the city works - guards, taxes, quarters, crime & punishment, well known leaders & NPCs - don't go nuts just some notes that will help you explain to the PCs what the city is like, where to go to buy stuff, etc. You don't have to detail everything just some headline stuff that you can build on session-by-session. It will make the city seem a living place.
Keep notes of all these random things you tell the PCs - names, prices, locations etc - they are bound to remember if you don't and catch you out (again! pesky players and their selective memories - can't remember the plot line but can remember how much a bucket of worms cost from John's Angling Shop in Fish Street...)
Finally, be prepared to ad lib even if you've prepped as above the PCs are bound to catch you out. Don't panic. Be ready to make stuff up just document what you say soyou don't forget and contradict yourself later!