You can increase your tax rate (in your castle) and/or build cottages (in your city) to increase your tax revenue and gain gold over time.
You can swap gold for happiness (or happiness for gold) in your Castle.
You can fire some knights (see “How do I use Knights” below).
You can sell resources for gold in your market (see above).
You can go to war and steal it (probably not a good idea when you are first starting out).