A dictionary-style transcription of pretty is /ˈprɪti/, with /ˈ/ marking the stressed first syllable, in both British and American sources.
In American English the t is often tapped, so the tongue gives a quick flick against the ridge behind the teeth and the word can sound close to priddy. British speakers keep an unaspirated t instead, so the sound is crisper and the tap never happens.
Say it in a phrase: a pretty day, keeping the first vowel like bit, not like bet.
How to shape the word
Spread your lips into a light smile for the first vowel. Keep the tongue high and forward, the same vowel as the i in bit, not the open e of bet. Say prit to lock in that first sound.
Touch the tongue tip to the ridge behind your top teeth for the t, then release it gently. The variety you follow decides the flavor: in American English, this t often becomes a quick, soft tap, close to a light d, as in the casual pronunciation of pretty sounding like PRID-ee. In British English, keep it a plain unaspirated t, as in tip or sit, released without a strong puff of air.
End with the tongue high and forward again for the final y vowel, the same sound as in happy or city. Lips stay spread, not rounded.
Now say the whole word smoothly: PRIT-ee. Stress the first syllable, keep the second short and light, then try it in a phrase: a pretty day, with the first vowel still like bit, never like bet.