A lots of Chinese language learners find it a great struggle to pronounce the four PinYin tones, well, you probably want to try the following tip to make it easier:
Yī 一（one) ---
compare this with the English word:
Mama (mama represents similar kind of tone, say it in a flat tone).
Yí 姨（aunt）--- compare this with this English word "what?", in a situation where you can't hear someone clearly, you go"what?", that is right, that is exactly the second tone in PinYin.
Yǐ 椅（chair）--- compare this with the English word "huh?", in a situation where something really doesn't make sense or is really confusing, you go"huh?" That is right, that is the third tone.
Yì 意（meaning）---compare this with the English word "NO!" in a situation where you want to give a very definitive rejection to someone, or you want to shout out an angry "no!". That is right, that is exactly the forth tone.