How do you get a horse to stand still?

i'm riding this horse who is green and i guess she never learned to stand still but when i have her all tacked up and enter the arena and stop in the middle to put down my stirrups she just trys to walk off and since i'm holding on the reins trying to halt her she just walks around me cause thats the only place she can go. another example is when i have her on a halter and lead rope and halt her at the cross ties she tries to keep going but since i'm standing still trying to halt her she walks around me and we have to do circles until she finaly stands straight. i'm sorry if im not clear in my description i'll edit if you don't undersstand