If you are in an accident at some point first aid or healthcare will turn up.
They will assess the situation and triage casualties. As more people arrive someone will get to you.
They'll do DRABC - check for danger, try to get a response from you. If you are unconscious then no response so they will check airway breathing and circulation.
If any of those are a problem they will take action - eg open airway by tilting head, do cpr etc.
By this point an ambulance is probably on its way.
At some point you'll be handed over to ambulance staff. They'll take a handover from the people on the scene and continue cpr if necessary.
You'll also have a top to toe survey done which basically checks for obvious injuries - broken bones etc.
When you get to hospital, they will deal with immediately life threatening injuries immediately.
(This is now a bit out of my area of knowledge but depending on what's wrong)
They will also do a survey of your body in more detail and will cut off clothes as necessary to access body parts (eg cut off legs of trousers to access ankle injuries).
They'll treat what they see and probably put in a line into a blood vessel to get drugs in.
When you are have been assessed and they have dealt with the immediately life threading stuff they will try to stabilise you and at that point you'll probably have a catheter put in at which point they will notice.