Not much to go on here, but there are two parts to filtering a dataset if you are using report-level parameters that you define yourself (as opposed to inheriting them from the dataset your report is based on).

First, you define the report-level parameters. (Right-click on the Parameters folder and create one.) Then you have to add a filter to your dataset. Right-click the Dataset, select Properties, Select Filters, then ADD. Then you can link your report-level parameter to the dataset-level parameter. Then you should be all set.