There are thousands of articles about railroad model scales so I will not write many words.
Just for quick reference, I plan to make models in the following scales:
|G scale||1:22.5||Commonly used for big garden raylways.
Read more on Wikipedia: G scale
|O scale||1:48||Not very popular but some manufacturers make models in this scale.
Read more on Wikipedia: O scale
|OO scale||1:76.2||Probably the most popular scale in United Kingdom.
Read more on Wikipedia: OO gauge
|HO scale||1:87||The most popular scale in the world (except U.K.). Almost all manufacturers produce large amount of models in this scale. If you did not decide in which scale to build a lyout then this is probably the best choice.
Read more on Wikipedia: HO scale
|N scale||1:160||This is probably the second most popular scale worldwide. If you do not have a lot of space for HO scale layout then N scale is good choice. There are many rolling stock stuff, building and decorations in this scale.
Read more on Wikipedia: N scale