English spelling is crazy, these two words are pronounced identically except for the first vowel. Don't worry about the 't' at all, it's not really a separate sound.
In Standard American English (the British and other varieties of English might have slightly different vowels):
'beaches' = beeeeeeee - chez, The 'eeeee..' sound, when exaggerated, is like when you clench your teeth and smile, very tense and long. In IPA, it is /bij tʃiz/
'bitches' = bih - chez. the 'ih' sound is very relaxed and short. In IPA, it is /bi tʃiz/
treffen WWY wer'" mar