Finding literature

Use Google Scholar
Use key words on Google Scholar to find relevant papers.
Snowball search
Once you've found a set of relevant articles, use them to "snowball":
- Check their reference lists for further works.
- Check the authors, see if they have other works on this topic.
- Check the conference / journal the article was published in. See if other issues have similar articles.
Keep PDFs
Sometimes, files become unfindable - even if you had found it before, even if you have the full title and authors. Therefore: once you obtain the PDF of a paper and decide its relevant, keep the file (in such a way you can find it later).
E.g. in a tool like Mendeley or Zotero, or just using well-structured paths and filenames on your hard disk. As long as you keep them.
Showing a hole in literature
Showing that a question has not been considered in literature cannot be done directly. The trick is to consider various related topics where the subject could/should have arisen. Now you can write about related work, and simultaneously support your assertion that the question has not yet been discussed in literature.