1
He has a bad attitude.
Sign 'attitude' while using a negative facial expression. Furrow your eyebrows, scrunch your nose slightly, or tilt your head to emphasize the 'bad' or 'sassy' nature of the attitude.
2
I need to change my attitude.
Point to yourself, sign 'need', 'change', and then 'attitude'. Keep your facial expression serious but determined to show your internal realization and intent to improve.
3
She always has a positive attitude.
Sign 'she', 'always', 'positive', and 'attitude'. For this context, make sure your face is bright, open, and smiling while signing 'attitude' to match the uplifting meaning of the sentence.