Although it has no full-text, Web of Science is perhaps the place to start: it is broad and multi-disciplinary, covers scholarly literature almost exclusively, and allows the searcher to see citation patterns. This last item, citation patterns, is important! This is an important tool for finding out which scholars and which papers are the influential ones.
Google Scholar also has a place in the starting lineup: although it can often retrieve too much, it is helpful for identifying keywords and terms for searching. If you're on campus, or using the VPN, it also provides links to full-text content (FindIt@MU links), and links to Web of Science.
The subject-specific databases can be very helpful too, because they will have filters/limits that help you find the most relevant results.
Click here for instructions on off-campus access.