Newbie question

Kristen • Mom to 👩🏻‍🦱👧🏼👶🏼

So, I’m just starting tracking my bbt, super late into my cycle but trying to get a jump start on next month. Is it necessary to use a bbt thermometer? Also, other than basic google, does anyone have any insights or know of reliable sources for info on how to properly interpret the information? Thank you all so much in advance.