You could use command such as top(for CPU, memory), du -h (for disk usage), etc on a regular basis using any monitoring tools like SeaLion, New Relic, munin, etc
New Relic lets you plot graphs based on these metrics if you would like to present it in the form of graphs with respect to time. It also has many other features you could use.
Since you are trying to provide the average, I would suggest SeaLion which gives raw data, allowing you to download the data for the period that you require and then "grep" on it to calculate the average for these metrics. Choose based on your requirement.