How can pilot remember when or where to do tasks such as changing radio frequency and change fuel tank?
Make your own checklists

While the manufacturer provided check lists are a good starting point and generally cover the operation of the aircraft well the FAA does not forbid you from extending them. If you have a few more steps you feel are worthy of a checklist write up and print your own. 

If you are switching tanks it sounds like you are flying a piper. Are you starting a time right before takeoff? If not you should be and you should always be changing tanks around the 30 minute mark. Another good rule of thumb with tank changes is to always execute them when near or overflying a field. Waiting an extra 5 minutes or switching 3 minutes early if their is a field below you is always a bit safer (should the engine cut out). Remember, for the pilot, a flight is not a journey from point A to point B it’s a series of maneuvers from one potential emergency landing spot to the next.

As a student pilot you are also still deloping habits which will be honed over time. You should debrief either with an instructor or do a self review after every flight. What went well what didn’t, what can be improved and how. The next time you get in the plane work these things and eventually you will build a good cockpit workflow.