From Grades > Setup > Categories and items you will see a list of all the activities in the unit's gradebook.
The maximum grade for each item is usually determined by the settings in the activity itself and cannot be adjusted in the gradebook. A manual gradebook item, however, can have a maximum grade set, especially if it is an assessable activity.
To adjust the maximum grade for a manual item, click the 'Edit' link next to that item, select 'Edit settings' and set the maximum grade as appropriate.
Often assessable items have different weights to the number of marks they are marked out of. For example, an essay may be marked out of 50, but is worth 35% of the total marks for the unit.
If the weights are different to the marks for any of the assessable activities in the unit, then it is important to assign weights to all the assessable activities.
If the aggregation for the category is 'Natural' then a check box will appear next to the weight for each item in the category. Tick each weight check box so they are no longer greyed out and assign each activity in the category a weighting, then click 'Save changes'.
You can also assign weightings for items that are not assessable, but these will not be counted in the assessable total, which is how the final grades for the unit will be determined.