I just got a 3 month old female retriever/pit bull mix and she won't walk. When we are in the yard, she is fine, stays somewhat near me, but when I put a leash on her, she won't walk. I tried putting a light chord(string) tied to her collar but she still sits and won't move. Today I dropped the chord and she pick it up in her mouth, turned around and walked back in the yard. She came from wide open space of grass and fields. I live in the city, not too noisy, but sidewalks and cars line the street. My plan was to walk with her every morning to my gym that is pet friendly and I can't get her past the neighbors house! Help!

I have friends who had a golden who wouldn't go near streets with traffic whenever I took care of him. He would glue himself to the sidewalk and refuse to budge, despite being an otherwise obedient dog. It didn't help that he weighed about 85 lbs. I don't think he ever had a traumatic experience with cars. He would obey his owners, but clearly didn't want to.
The only way to do this is with patience and reward, and a bit of firmness. Use the leash, keep her close to your side, pat her when you get to a corner, give a random treat but not too often or in any pattern, and huge hugs and pats and tummy rubs and praise when she gets to the next stage. Even though she came from a rural area it doesn't mean she didn't have a close encounter with a vehicle.

First of all, your puppy is only 3 months old. I highly doubt that she has fully completed her round of vaccines. She needs 3 puppy shots before she can safely walk from point A to point B. Parvo is not a fun thing to deal with and it can be very pricey.

Instead of introducing the leash outside where there are tons of distractions, put a leash on her in your living room and for 5 Minutes A DAY just put gentle but consistent pressure on the leash and collar until the puppy moves in the direction you want. Give treats and praise at this time. The easier it is for her to move in small steps from one place to another, than graduate to walking down the hall or in various rooms of the house. THAN once that is going well, you can start heading outside.

This is a puppy, not an adult. The World is very scary still.

Please wait until she has completed ALL 3 puppy vaccines before you consider walking her down the street and into a parvo infested doggy daycare (Your gym).

I agree 100% with Lucky.

So many people think that all dogs are born knowing how to walk on a leash. They aren't. It takes time, patience, and training, to get a dog to successfully walk on a leash.

The first step is introducing the collar. Once puppy has accepted the collar, you introduce the leash. You do both slowly, and for only a few minutes a day, increasing the time every day. The best way to start, once puppy is okay with having both collar and leash on, is in your home. Tie the leash around your waist and let puppy follow you around. When he does, lots of praise and a treat. It's going to take time, it won't be instant. Your puppy has to learn to be the pet you want him to be, it's all new to him, and it's going to take time and patience on your part.