I struggled a lot about writing accents and special characters in MacOSX with an italian keyboard. The most comfortable way to do them is to hold option+ press a key

Specifically, for tilde, as mention above is option+5 (try other keys also, you'll be used to it in a while)

there are other special combinations for accents which are 
option+n and a letter for ñ
option+u and a letter for ü
option+8 and a letter for ú
option+9 and a letter for ù
option+ì and a letter for ô