Obtaining Tcl/Tk`
Look at places to find Tcl binaries and sources or the ones listed here for your convenience:
Binary distributions for common platforms
Source code
Note that the Tk released files are under the Tcl Project, not the Tk url given above.
Extras you should have
Learning Tcl/Tk
Once you've got Tcl/Tk on your computer there are a number of pages that will help you learn about Tcl and Tk
Getting deeper
There are also a number of books available
Also available is the Tcl Tutor program and various Online Tcl and Tk Tutorials.
Web resources worth visiting include
Advanced resources