If you approach the acquisition of antique toy trains as a potentially fun and rewarding experience, not as a competition or simply an investment, you will save yourself a fair amount of beginner's anxiety. The key is to spend time reading and researching a particular niche and then plan on continuing that investigation for as long as it is fun and interesting. For example you can find plenty of generic information regarding the collecting of toy trains, but you must decide what aspect is of particular interest to you. Let's look at pre-war Lionel as an example.
Pre-war Lionel encompasses the period from 1900 to 1942. It is further broken down by 4 gauges, which represent the measurement between the center line of the two outside rails on a piece of toy track. The pre-war gauges are 2 7/8, Standard , O and OO, in descending order of measurement. Gauge should be further distinguished from scale, which refers to a comparison of the model to the prototype (actual locomotive or piece of rolling stock). In the early years, when the Lionel production was more toy like, scale was not particularly accurate. However, in 1937 Lionel made its first 1:48 Scale Locomotive, the O Gauge #700E Hudson, catering to the model railroader rather than the toy market. For the beginner it is helpful to have a basic understanding of the above to have a sense of proportion, but remember your early interest as a buyer and collector is purely aesthetic. Buy what you like, the specifics will come with experience.
Now you may say, how do I satisfy my early interest without getting taken? Fortunately there are many aides to help you avoid financial pain. First of all there are numerous price guides. Kalmbach Publishing Co publishes the Greenburg's Guides to Lionel Trains. It is readily availale oneBay or at your favorite train store and is a must for the beginner. Secondly there are many books such as the Standard Catalogue of Lionel Trains published by KP Books that provide prices and pictures, along with guidelines regarding grading standards. Finally there are organizations that you can join populated by experienced collectors such as the TCA or TTOS which establish standards for sellers regarding condition and grading.
Well it's time to provide the secret to your successful first transaction. Don't procrastinate and say I'll just wait another week and then I'll make my first bid. Dive in and have fun! Just stay loose. Pick something simple and inexpensive in a particular niche, lets say O Gauge, tinplate between 1930 and 1940, like a Lionel #248. That was my first locomotive purchase on eBay and I still have it and enjoy it. My personal collecting goals have changed with time, but the interest hasn't lessened one bit. Hope this helps and most of all I hope you have fun participating in "the world's greatest hobby."