Codes for counting

I have a spreadsheet that needs to track different activities but if I put formulas in, the calculations will make the workbook slow and a pain on the network. So I figured the best way was to program it in, only I don't have a clue on how to write it. I have attached a sample and if you can think of something better, I'm open to suggestions.

There will be a total of 26 people entering the same type of information into this workbook. This is what I would like to happen.
1.When someone enters information on the activities tab,
a. I would that information to total up onto the YTD Summary by SC tab, in columns C - AO matching of course to the region, sc and activity.
b. I would that information to total up onto the Monthly Summary tab, in columns B- M for the activities that fall within the month from column a on the activities tab.
2. When someone enters information on the cookout tab,
a. I would that information to total up onto the YTD Summary by SC tab, in column AP, matching of course to the region, and sc.
b.I would that information to total up onto the Monthly Summary tab (row 48&49), in columns B- M for the activities that fall within the month from columns C & G on the cookout tab.
3. When they enter a number in column I on the activities tab, I would like a pop up box that says "Don't forget to enter detailed information into the collision review database".

Thanks in advance for your help and your knowledge.
Michelle