I've been learning about UML diagrams and such. While doing so I came up with a problem. Which diagram should be drawn first when designing a system given that use case and such are completed? Is the activity diagram drawn before the sequence diagram or after?
There is no order for the creation of diagrams, of any type. Depending on your project, you may not even find use for some diagram types. That said, I would expect that activity diagrams, if created, would be created earlier than sequence diagrams. Activity diagrams represent workflows and processes and show sequence and concurrency of steps, and do so without showing design details of modules, classes, and methods. Sequence diagrams typically show the interaction between classes, instances of those classes, and the method calls (with arguments). The information shown in an activity diagram is known earlier in a project and is independent of things like technology choice, which may change how you consider the items shown in a sequence diagram.
There is no order, but I like to draw Activity Diagram first. Activity Diagrams make me know and understand all the possible Successful scenarios (Happy path) as well as unsuccessful scenarios.