depending on what language you are using, the implementation for this will differ.
For Java projects, I would suggest looking at the Custom Code->tSetGlobalVar component.
you can use this component to set the values of a variable, or access the global map directly
with globalMap.put(String Key, Object Value) and globalMap.get(String Key)
For Perl projects, all variables are global in scope, so you can create one in a tPerlRow or tPerlFlex and use it elsewhere.