How about:
$thumb = get_post_meta( $post_id, 'thumb', true );
$url = get_post_meta( $post_id, 'website', true);
echo "<a href="https://wordpress.stackexchange.com/questions/24970/$url"><img src="$thumb" /></a>";
The get_post_meta() function is ideal when you know ahead-of-time the names of the fields you want to retrieve. Setting the third parameter to true tells it that you want to return a single value, and not an array of values (it’s possible to set multiple values using the same meta key, and get an array of values back).