It's recommended that you create an account at Ensembl. This allows you to save searches, upload custom data tracks to the Genome Browser, save VEP (Variant Effect Predictor) results, and much more. Registration is free. Click the "Login/Register" link in the upper right corner of any Ensembl or BioMart page to register or login to an existing account.
When you enter the BioMart search page, the "New" button is highlighted (toward the upper left). As you search, you can always abandon your current query and start a search by clicking on the "New" button to start over.
To start, you select a database from the "CHOOSE DATABASE" drop-down menu.