Use native classes Prestashop to make a purchase

[ucip3]

I want to make an external script where I pick up the 'id' of the customer and products to buy

 

I want to see if the client is connected and if it is not connected, connect it.

 

Create a new shopping cart to any product and confirm the purchase.

 

What classes of having prestashop should I use? What are the specific methods?

 

Any information would be helpful.

 

Thanks in advance