Can someone please tell me how to be less anxious??

Seriously, because i get borderline anxiety attacks or otherwise mental breakdowns from simple human things. 

Like, I got a mocha frappe at McDonalds today, ordered AT THE KIOSK and just had to pay at the register since I only had cash. I completely fumble and overthink eveything as my heartrate kicks up and I struggle to form a coherant sentence or thought. 

Another situation, I'm literaly just walking outside and my mind convinces me that a man I see has a firearm and is going to kill me. I completly freak out and stress over that. Then, when I go to a store, I coincidently see this guy again so now my brain is completely convinced that he is out to get me. Truly terrible experience.

Thirdly - literally just standing in a crowded line at school. Felt like I was suffocating, I struggle everytime to not have a complete breakdown.

So, help T-T