I've been asked to give lectures on electromagnetism in English, but I encounter many problems trying to express mathematical formulas since they are written and I do not know how to read them. Are there any references which elaborate the formulas and express what should we call any mathematical sign?
The following references are useful:
There's a certain amount of variation in how signs are denoted in speech, not least that given signs may have a different meaning in context (simple example: a superscripted number is usually a power, but it might not be).
You're best asking a colleague who is a mathematician who was educated at, or has a long association, with the institution where you will be lecturing, and knows the standard conventions there.
If you're concerned about this, I am afraid you are likely to bore your audience to tears. Describing mathematical equations orally is rarely effective. You would be much better off to provide a hand-out with a numbered list of all equations you wish to discuss, and refer to them by number.