Andy has a video on the site called 'state shifting' which you should look at- but its basically a way to warm up. First of all, walk into shops and talk to staff members to get in that talkative state. Once you have doen this then you could try stopping random people and asking for the time or for directions.
Then when you feel you are ready you could focus on just giving throw away compliments which means stopping a girl and telling her you are on a rush somewhere but you just had to say that she looks amazing today etc. This way there is no pressure on yourself to stay in set and carry on talking to her.
Once you are happy to do this then force yourself to stay and make general conversation.