A/C will not blow and then it will

I got in my car today and my A/C would not turn on at all. I drove a mile down the road and all of a sudden it started blowing out cold air. Then I shut it off went into the store came back out and it would not blow out any air. I went to AutoZone and O'Reilly's. I got different answers from both people. Pretty much they said it's either going to be my relay maybe your sensor or my blower motor. When it does come on it blows out cold air but if I hit a bump or when I come to a stop the air goes to my high speed to low speed. Does anybody have an answer for me on what I actually really need to get fixed?