What are the basic commands I need to know for sending mail and postings?
To start composing a new mail hit ‘m’ either in Group or Summary buffer, for a posting, it's either ‘a’ in Group buffer and filling the Newsgroups header manually or ‘a’ in the Summary buffer of the group where the posting shall be send to. Replying by mail is ‘r’ if you don't want to cite the author, or import the cited text manually and ‘R’ to cite the text of the original message. For a follow up to a newsgroup, it's ‘f’ and ‘F’ (analogously to ‘r’ and ‘R’).
Enter new headers above the line saying "–text follows this line–", enter the text below the line. When ready hit ‘C-c C-c’, to send the message, if you want to finish it later hit ‘C-c C-d’ to save it in the drafts group, where you can start editing it again by saying ‘D e’.