Tilde: this character is located to the left of the number 1 key. To type ~ hold the right Shift key with your right pinky finger and press its key with your left pinky finger.
Underscore also called underline, underbar, low line, or low dash: this character is located with the Minus key. To type _ hold the left Shift key with your left pinky finger and press its key with your right pinky finger.