I suppose, the best way to learn ANY language, is to study its alphabet at first, then take any book of the native language author, take a vocabulary, and start reading the book with vocabulary; it all supposes you work with such materials manually with a pen, without computers. Also, watching movies with subtitles is a good way.
Despite there are strict rules regarding Latvian usage in the working and studiying places, you will rather not find an online website providing free Latvian courses fir English speakers; at least I don't know any of such websites.
You still may start with valoda.lv
, the only reference developed so far by the Latvian Language Agency. Check the ENG link in the top right corner of the menu, and you will be forwarded to the English version of this website. Then click Teaching And Learning Latvian
, select UK flag at the top menu (supposing switching to English again) and you will open an online learning section with useful links.
If you have more questions, feel free to ask!