Immediately under the "Issues" tab, there is a search box that can be used for searching. Each search takes place in a search context that is selected by a drop-down menu. By default, all searches only consider open issues; if you would like to search closed issues, select "All issues" from the search context drop-down menu.
You can quickly narrow your search by clicking on the issue list table headers and selecting a value from the "Show only" sub-menu.
Another way to search is to use the "Advanced Search" link under the "Issues" tab. This page helps you form a complex query. However, once you learn the query syntax, you can enter complex queries directly into the main search field. For more information on issue queries, click "Search Tips" under the "Issues" tab.