In my forum I search for the word "cat" and for the word "giraffe".
Do you mean that when searching, you specify these two words at once
, or do you searcgh separately for each word? Not quite sure I understood your question.
How can I search in both FIELDS, TOPICS and MESSAGES?
You could only search in Titles or Messages, not both at once. In the most recent versions of miniBB, if no results are found in some field, you will be suggested an auto-link to search in the other field.
Titles are the most important on the forum, so you should care about their quality at first, boosting them with all the top keywords which could be used in search to find them.