Footnotes: is there any 'rule' on how to use them?

I am writing my Msc thesis and I would like to know if it is ok to use footnotes and if there are rules about using them (maybe related to the text format or other issues about the style, for example, should I use numbers or letters?) And are there guidelines about what should not be put as footnote?

Sorry if this might sound as a silly question, but since it is a thesis and not a book (therefore the aim is presenting results and not teaching people) I do not what it to look presumptuous