You can create a parameter query to run the report from. This will provide a prompt box for the user to enter something into. Beware though, the input is free text so any incorrect characters will result in an empty report.
The other way is to have a combo box on your form where the user selects a value and the then clicks the command button. You will need to write some code to pass the selected value of the combo box to the report ie:
Assumptions:
1. Report is a monthly report called"MonthlyReport"
2. user needs to select a month in a combo box called "cboSelMonth":
3. Field on the report to filter on is called "Month"
in the OnClick event for the command button, enter the following code...
docmd.OpenReport "MonthlyReport",,,"Month='" & cboSelMonth & "'"
If the month selected in the combo box is May, the "WhereCondition" for the report will be Month='May'.
Your report will open only showing the data where the month field contains the word 'May'
Hope this helps...