Basically, in order to get past the List View Threshold, you need to specify a filter on a column that is indexed (and returns less than 5000 items). If a column is not indexed you can choose to index a non-indexed column.
Excerpt from Office.com:
For a view to quickly filter through a large number of items, the
first column that you specify in the filter must be indexed. Other
columns you specify in the view filter may or may not be indexed, but
the view does not use those indexes. You also need to make sure that
the first column of the filter does not return more items than the
List View Threshold, even if the final result of the filtered view
returns less than the List View Threshold.
If the first column of the filter returns more items than the List
View Threshold, you can use a filter with two or more columns. When
you define a filtered view that uses two or more columns, using an AND
operator will usually limit the total number of items returned. But
even in this case, you still need to specify as the first column in
the filter the column that most likely returns the lesser amount of
data. Note that using an OR filter almost always increases the number
of items returned and won't be effective under these circumstances.
To create an indexed column (Except here):
- Navigate to the site that contains the list or library.
- Click its name on the Quick Launch, or click on the Settings menu , click View All Site Content, and then click the name.
- Click List or Library > List Settings or Library Settings.
- Under the Columns section, click Indexed columns.
- On the Indexed Columns page, click Create a new index.
- Do one of the following: To create a simple index:
- In the Primary Column section, under Primary column for this index, select the column.
- Click Create. To create a compound index:
- In the Primary Column section, under Primary column for this index, select the column.
- In the Secondary Column section, under Secondary column for this index, select a different column.
- Click Create.