Select the most important 2-3 terms related to your topic. Put the word AND between each new term; this tells the database to look for resources with all your terms.
food waste AND apps
Use quotation marks ( " " ) around phrases (two or more words); this tells the database to look for the words in that specific order.
"genetically modified organism" AND food
Use OR in between synonyms, alternate spellings, related terms, etc.
GMO OR "genetically modified organism"
If you don't get good results, try again. Change up your keywords - try synonyms and related terms.
Use database filters, Full Text, Source Type, Publication Date, to help you limit to the types of sources your are looking for.