Ok, with a little more data I can help a bit more.
I don't use Macros so bear with me (if you're brave, I'll list the code way to do it at the end).
Your form needs to have a field on it somewhere that shows the Primary Key for that record. It can be invisible and hidden, it just needs to be there. Then when you go to your macro to print the report, the Where clause will be something very similar to this:
WHERE Condition: [PKFieldInTable] = Forms![FormName].[PKFieldOnForm]
The parts in brackets need to be adjusted to match your form/field names. (I hope I did that correctly per Adrianna's example below)
If you code the button directly, the command is simply:
DoCmd.OpenReport "[ReportName]",,,"[PKFieldInTable] = " & Me.[PKFieldOnForm]
Both of these assume that your PK is numeric. If it's text, you'll have to enclose the reference in single quotes.
...= '" & Me.PKField & "'"
Because you're calling it directly from the form you're referencing, the 'Me.' takes the place of explicitly naming the form.