The concept of a design pattern has been taken from structural engineering, as with many practices in software engineering. If you consider building a structure, there are decisions that need to be made on how to build that structure to achieve the goals set out. When making those decisions, you will have a set of requirements to work from. It may be something as simple as Bridge must be able to support X tons at one time, or have a particular tensile strength to allow enough movement in wind etc. An architect would use prior knowledge of other builds to make those design choices. He/She would be very unlikely to try to solve the problem from scratch.
Software Engineering and Design Patterns are exactly the same. They are simply common solutions to common problems. If you know the design patterns, then when you are working through a design, and particular part of a system requires something that fits a design pattern you have, then use it. Don't try to fit a system round a design pattern, fit design patterns in to your system (where they fit). Just try to think of them as a set of solutions to reduce the amount of design work you need to do, and be cautious of over-engineering your solutions to cram in as many design patterns as you can. This will just serve to make your solution unmaintainable and probably quite buggy.
