I am new to LibreOffice. I am finding it very frustrating to find out how to adjust the margins on a word-processing document (letter, etc.) and to find the tool for addressing envelopes.
To adjust the page margins of a document go Format (menu) > Page. . . > Page (tab) There you will see the margins you can set.
Don’t know how to deal with your addressing envelopes issues. However, there is a item Insert (menu) > Envelope… which probably offers what you are looking for. I’ve never used it so can’t comment on how to.
You might find the following two books useful:
Both books are free unless you want a hard-copy.