In order to be a good investor you have to be knowledgeable, create a clear plan and execute each task, even if it feels like you are jumping off a cliff. As Nike says "Just do it".
The following tools are the top of the line tools necessary to invest in New York City. Some of these are exclusive to New York City and most likley may not apply elsewhere.
Zillow.com
Every real estate agent hates Zillow. But don't worry about that. Zillow is a great tool to research recently sold properties. Although it appears credible, it is NOT a good tool for doing rental or purchase comps. Again, only use it for researching properties that have been sold.
OASISNYC.com
This is a magical web site. I couldn't be a real estate investor in New York City without it. Use the map feature to do a property search. Once you ID the property, use the menu in the right hand column to look up property information. Everything form fines, to real estate transactions and tax records are available. It's awesome.
There are others similiar to Zillow including Property Shark and Trulia.com but for ease of use Zillow is easiest. Again, you are using it to comp recently sold and for sale more than anything else. You can't rely on the estimates they give you.