No one index includes all journal articles so you'll need to search multiple databases. There are three main databases to search:
Studies reporting negative results are less likely to be published in journals than those reporting positive results. Including unpublished studies and other grey literature in your review reduces publication bias and may affect your statistical results.
search.bioPreprint
Searches multiple preprint resources at once including: medRxiv, bioRxiv, ChemRxiv, AAS Open Res, F1000Res, Gates Open Res, HRB Open Res, MNI Open Res, PeerJ Preprints, Preprints.org, Research Square, Wellcome Open Res.
medRxiv
A preprint server for medical, clinical, and related health sciences.
bioRxiv
A preprint server for the Life Sciences, including Cancer Biology, Immunology, Pharmacology and Toxicology.
PRISMA-S Checklist: a guide on how to describe your search strategies in your manuscript
PRISMA-S Checklist: a PRISMA checklist of items to include when reporting literature searches in systematic reviews
Standards for Finding and Assessing Individual Studies (Ch. 3 in Finding What Works in Health Care: Standards for Systematic Reviews): instructions for identification, screening, data collection, and appraisal of individual studies
"Searching for and Selecting Studies" (Cochrane Handbook Ch 4) Search guidelines for authors of Cochrane systematic reviews. It gives an overview of sources to search, how to plan the search process, design search strategies, manage references, and document the search process.
Hedges are search strings created by expert searchers to help you retrieve specific types of studies or topics.
Yale MeSH Analyzer: Enter up to 20 PMIDS to see a list of MeSH subject headings.
PubMed PubMedReminer: Enter multiple PMIDs to see a list of MeSH subject headings. Also lists authors, journals, and countries.
NLM MeSH on Demand: Enter a paragraph and find the most likely MeSH subject headings to search for additional articles.
Polyglot Search Translator: Translates a PubMed or Ovid Medline search string to multiple other databases. (Note: translates the format but not the subject headings)
CitationChaser: enter in an article and get a list of all of the references in it and all of the later articles that cited it.
Before you start a systematic review, it's a good idea to see if one has already been published on your topic. There are several places to find them:
Searches ten different databases for systematic reviews.
Search for your topic and select "Systematic Review" in Article Types on the left.
An online registry of systematic reviews currently in progress.
Detailed descriptions of Cochrane systematic review protocols.
Before you start a systematic review, it's a good idea to do a quick search to get an idea of what kinds of studies and how many studies have been written on your topic.