How shall I introduce my puppy to walks?

I have a border collie x gsd and she is very weary of people when she first mets them, it's not long untill I can walk her, how much exersize should I give her?? Shall I put her on an extendable lead at the park?? And will she be ok with older puppys, she's almost ten weeks and the size of a beagle, and she gets on well with my king Charles, no rude answers please, just advise on walking sheep dog breeds, thanks.