How to set stuff like From, Organization, Reply-To, signature...?
There are other ways, but you should use posting styles for this. (See below why). This example should make the syntax clear:
(setq gnus-posting-styles '((".*" (name "Frank Schmitt") (address "firstname.lastname@example.org") (organization "Hamme net, kren mer och nimmi") (signature-file "~/.signature") ("X-SampleHeader" "foobar") (eval (setq some-variable "Foo bar")))))
The ".*" means that this settings are the default ones (see below), valid values for the first element of the following lists are signature, signature-file, organization, address, name or body. The attribute name can also be a string. In that case, this will be used as a header name, and the value will be inserted in the headers of the article; if the value is `nil', the header name will be removed. You can also say (eval (foo bar)), then the function foo will be evaluated with argument bar and the result will be thrown away.