To begin to teach your dog to walk nicely on the leash, go outside in a fairly boring area with your dog on a leash and with treats in your pouch. I suggest mixing in some really tasty fresh treats like cheese and the treats should be small (pea-sized is great). My favorite tool to start with for leash training is the Freedom Harness.
Now that you are ready to go, you will start with working on attention outside before you even begin to walk. Just wait for a bit with your dog on the leash and if she looks at you say "good" and feed a treat. Do this a few times while just standing still. Now say, "let's go", and go a few steps by luring your dog with a treat in your hand held to her nose so she follows it to walk along by your side. Take a couple of steps like this and then say "good" and step in front of your dog, turn to face her and then lure to sit and feed the treat.
Say "let's go" and start again and this time just show your dog the treat, but do not lure. If your dog makes eye contact with you, say "good" and step in front of her again like above t to feed the treat.
Do this walking back and forth in a boring area at first. If your dog is not automatically looking at you, try saying your dog's name so that he/she looks at you and you can say "good" and step in front to feed a treat.
If your dog is not very food motivated, try using a extra special treat like small pieces of string cheese or hot dog. If you are still losing your dog's attention, try practicing in a more boring area and/or rewarding more frequently. If your dog is having a LOT of trouble focusing, try just stopping and waiting for her to give you some attention and calm down a bit before starting again. Leash training is challenging because it is very distracting for dogs to be outside.
If your dog walks past you using this method, lure her in a 360 around to get back in position to start again.
Cue "go sniff" every few minutes as a break and allow your dog to sniff or potty but if she pulls do not move forward (to prevent rewarding pulling).
For safety, do not allow your dog to cross in front of you by using the leash pressure to prevent it.
Once your dog is walking in a nice position by your side reliably, you can simply feed a treat when your dog looks at you but you do not need to step in front. Then you gradually reduce the treat frequency over time but noting to feed more in more distracting environments.
Tips: If the above method is not working and your dog has a habit of rushing ahead of you, you can try using the leash and your body to block her from rushing past you by stepping in front. Also, If you need to just do a non-training walk, it can help to use a different leash attachment (like attaching to the back of a no-pull harness instead of the front). And finally, for severe pulling that feels like you may lose control of your dog it can be useful to teach your dog to wear a Gentle Leader.