In general, if an experienced programmer knows one language, they can readily pick up another with no difficulty. Depending on the size and complexity of the language, they can learn the syntax and basic feature set in a matter of days or weeks, but to appreciate the programming idioms of the language will require some practical experience using the language to write software.
In the case of Smalltalk, since it’s so ridiculously small and simple, you can pick up on the syntax in no time, but you should spend a couple of months writing a non-trivial program to really “get it.”
There are a number of fine Smalltalk dialects, but the one I usually recommend is Pharo. It’s the most actively developing Smalltalk today and it has the largest user community. Pharo was originally a fork of Squeak, and Squeak is also a good choice (esp. for the education market). If you’re a Windows user, you may want to give Dolphin Smalltalk a serious look.