A planner will give you one place to keep all of your notes, dates and times. Planners come in all sorts of configurations from daily to weekly to monthly, or any combination of these. In a combination weekly/monthly you can view each week, with big blocks of space on each day, as well as a month view to look at your whole month at once.
Now that you have a planner, write everything down. Make sure you take that note about homework and when that paper is due. Write everything down. Without this, your planner is worthless.
In addition to writing everything down, color code it. Use different highlighters or different color pens to give each class or event a different color.
For example, use red for due dates for assignments, blue for class times, green for extracurricular activities and purple for your work schedule.
Keep your planner with your schoolwork, or in your book bag. Bring it to every class. If you forget it, write down your assignment on the front of your notebook in a highly visible place and transfer it to your planner as soon as you are reunited with it.
Keeping your work is important. Keep hard copies and digital files. Keep them backed up on an external hard drive, and in a file cabinet or folder. File according to class and keep until your final grade is posted for each class. You may even want to keep some of your work beyond class, in case it will help you with your projected career.