If you use the search term, "missions," the online catalog will return approx. 91 pages of results. If you repeat this search in StorySearch, you will get over 25 million results. You may want to narrow down your search, so what/where/when/how/who about missions do you want to study? You will want to combine some terms, i.e. "missions" AND "papua new guinea" or "missions" AND "women". Think about your topic, area of focus or interest, and narrow your search terms to fit that.
Please look at the links I'm attaching. They will point you in the right direction: access to our catalog and StorySearch, and some searching how-tos.