Navigate to the reports Object/Tab and use the Search button to find the report you would like.  If you know where the report is you can look in its folder, or if you used it recently, you can use the Recent Reports tab.  Click on the report to take you to the Run Report WIndow.  In this case we are going to use the All Users - Demo report.  Then click customize.

Salesforce has wonderful training and lessons on how to use Operators, Fields, and Values when working with different types of Fields.  

Find out more at : Salesforce Help and Training: Filters

In the Filters Menu of the Customize Reports window, click on the Edit Custom Filters.  This will open a new window.

I don't need the Profile to be limited to only Volunteer and Volunteer Old, so I am going to remove this filter by hitting the Remove button to the right of this filter

SO, for this report, I want men who are over the age of 55.  That is going to be two filters.  One on gender, and the other on age or birthdate.  Let's add the Gender Filter.

From the Drop box, let's find Gender.  It is in the Contacts Object and I just scroll through til I find it.

In this case, we only want to bring back records where the Gender is (or equals...) Male.  You can see the choices we have here... (We will use greater than or equal in the next filter)

If the Field you are choosing to Filter by is a Picklist or Mulitpick list, you can use the Lookup Magnifying Glass.  This is especially helpful when you want to filter by Organization Name.  You can use the Lookup to find that field.  If the option to use this Lookup is there, I suggest you always use it.  We are going to put a check box next to Male and hit Insert Selected.

You will that once we start to add a filter, it adds an AND line. This is where you can add another filter.  Filter order does not matter.  I am not using the field Age so that it is greater than or equal to 55.  I could have also used Birthday and said Less than or Equal to 55 years ago today.  Age, however is a calculated field, and if I save this record, I would have to adjust this filter every time I ran it.

When you check the box next to limit rows, you will get other options.  If I used these filter logic,  it would bring me back the 3 oldest people in the report.  Why?  I have said I only want three rows, I want them sorted by Age, and I want them in descending order.   NOTE:  The Sort by picklist only lists the Fields that you have in the report.  Back in the Customize Report Window I added the field Age.  If you don't' see the field you want to sort by, you need to add it into your report.   Now Hit OKAY

Just above your sample data (remember, this is just a SAMPLE...not the records you are asking for...), you will see all the filters you have applied.  Hit Run your report.

This will take you back to the run report with the records you have requested according to your filters.  Note that your filters are listed here as well.  You can always edit your filters through the Edit button (1) or remove filters (2)

Use save to overwrite the existing file, or Save As to create a new report with the options, layout, and filters you have created.

In this example, I want my report to pull back information where the City field is not NULL.  The inverse is true, as well.  If I wanted to pull back information where the City is blank, I would create a filter where the Field is City, the Operator is equals and in the Value box, leave it blank.  NULL means that there is no value in the field... that is very different than a field that is filled with the alpha value: None or No one.  

You can create logic between your filters to create AND / OR logic, or limit the number of rows of information returned in the report.

See Salesforce Help for more details on advanced features.

See the next post to learn about cross-filters