1. Stop and stand still
Each time your dog pulls, stop and stand still until your dog looks back at you. Keep repeating this process each time you walk (it can be a little tedious but it is effective). Remember to praise your dog when they do not pull, this will help them understand what you require from them. Try saying 'good boy/girl' in a spritely, happy voice so they know you are pleased with them. They are more likely to understand this is positive, as the tone of your voice is more important that what you say.
2. Try a front lead harness
A front D-ring harness, where the lead can clip at the front gives you more control than a regular harness or collar. If the dog pulls it twists slightly to turn their shoulders and chest to you, discouraging them from pulling. You should always ensure your harness fits correctly first.
3. Spend time treat training
Dedicate time to training this specific skills by getting some delicious treats in and teaching your dog that no pulling means a reward. To do this, try walking up and down your garden on lead, and begin with one step, treat, two, and so forth. Build up the amount of steps you can take with no pulling, and be sure to give them a big fuss by verbally praising them too.
4. Teach 'Heel'
Go one step further and teach your dog heel, but doing the same process as above but off lead.
Be patient, and do it step by step. Remember to reward and praise your dog, and use the cue 'Yes' to let them know that (A) they've done what you want, and (B) to signal they will get a treat imminently.