Collies are not guarders at all they are herders - it is a totally different thing.
@ThisisMax Initially you need to get your dog to focus on you. Loads of rewarding eye contact whilst stationery and when on the move.
Introduce your dog to a clicker or marker word eg mark the behaviour and reward/reinforce. This is important that the dog understands the clicker and what happens.
When under threshold have the dog look at person/dog/car and click the clicker and feed the dog whatever the dog does. You may have to take food down to the dog. IF the dog does not eat the food the dog is overthreshold and you need to increase the distance. If the dog is barking lunging, or over staring they are too close .
Initially the distace may need to be huge eg across the other side of a football pitch.
Very quickly if you click and reward every time the collie sees something they will look back at you for the reward. At this point you can say Look at that. From this point on you can say look at that the dog will look and turn to you.
The dogs emotional response to the trigger has changed and their behaviour to the trigger has changed.
Do this on things the dog does not react to as well as things they do react to to prevent look at me becoming a precursor.
You can over time decrease the distance so that you can now walk past anything that you want to - if you want your collie to interact with dogs it is now easier for them to do so as they are in a calm state at meeting and greeting.
Your collie now has a clear job to do when walking past things, seeing things that nove etc.
If you want more info look at Controlled Unleased book by the amazing Leslie MCDevitt