Equivalents (or better, compatibles) are hard, because whether tube XYZ will work instead of tube ABC really depends on the circuit. Best way to get started is to hang out on message boards (like this) and see what other people are using. The Valve Museum tends to list "sensibly equivalent" tubes, although many are obscure.
http://www.r-type.org/exhib/aaa0540.htmIncompatible is much easier. You can google "XYZ tube data" and look at the datasheet and compare e.g. pinout and heater current and voltage to "ABC tube data."
The RCA Tube Manual is a great reference if you are interested in tubes. But even it is a little confusing because there are different editions that focus on the current "tubes of the day." In other words the 1970s edition is going to be different than the 1940s. But it has a ton of good information even beyond specs for individual tubes.