Utilize your local libraries and information centers. Depending on where you live, they may have access to a large number of databases and document registries. In addition, they have a number of print sources on a multitude of topics, true crime included.
Never, ever pay for an article. Ever. Do not buy a subscription to use a database. Many times, if you find an article that you are interested in using, you can simply email one of the authors (best to do the lead researcher) and ask if you may have a copy of it for research purposes. The authors are usually not beholden to any of these databases, so they can give the article away as they please for free. I've done this a number of times and have only been turned down once or twice.
Read. Read a lot. Take plenty of notes. Make sure your notes have some direction. Know going in what you would like to focus on. If you cannot know the direction going in, try to find one quickly as you read.
If you're focusing on online individuals, then make sure any sources you find are legitimate. Try to find other sources that corroborate each other. If there are any contradictions, these need to be investigated.