Buy your milk in plastic bottles and not in cartons, but take the empty bottles to a recycling collection centre.
The cartons (bricks) are in plastified cardboard, requiring a lot of energy to make, and are impossible to recycle. They help fill the landfills.
The bottles are in a polyester called polyethylene terphthalate (PET). They are not recycled back into bottles for foodstuffs, for hygienic reasons, but they are recycled back into bottles for other products, packing materials, fibres for insulation, knitting "wool", clothing, carpets, ropes etc. To recycle used bottles into any of these consumes about one-tenth of the energy than it takes to manufacture virgin plastic and no petroleum.
However, before offloading them for recycling, I have used them a second time, as freezing containers for liquids, such as soup or fruit juice. Immediately after they are emptied, I half-fill them with cold water, put the lid on with a good shake and repeat, to clean them. I've used them frequently for home-made lemon squash, veggie soups, grapefruit and orange juice etc. Be sure to leave about 4-5 cm empty at the top or you may have a mess in your freezer, as the products expand on freezing. I've frozen soup which was perfectly OK after more than a year.
I re-use the bottles only once before recycling.