The worst thing you can do after spilling liquid into a laptop is to try to turn it on. This can cause electricity to flow to places where it shouldn't, frying circuitry. It isn't enough to just turn the laptop off. You have to disconnect the battery. Easy if it's an external battery. But if it's one of the modern kind with an internal battery, you have to open it up and detach the wires connecting the battery to the motherboard.
The second worst thing you can do, if it's any liquid other than distilled water (e.g. rain), is to to let it dry. Once the water evaporates, the crud that was dissolved in the water (minerals for tap water, various biological components for milk, sugar and other chemicals for soft drinks) hardens into a crust over the electronics. It can short out circuits, and is frequently difficult to impossible to get off without physical scrubbing. (Salt water is similar, but in a whole league of its own. If you spill salt water on electronics, you want to keep it submerged until you can get it to a fresh water bath. Salt water + access to oxygen in air = corrosion galore.)
Since it was a substance with a lot of dissolved particulates (compared to, say, tea), in addition to the tools to open up the laptop, you're gonna have to buy a few gallons of distilled water and a bottle of 99% isopropyl alcohol. Open the laptop and disassemble it best you can. Bathe and rinse the components (not the screen or the HDD, probably not the fans unless they've got milk all over them - the copper wire windings for the fan motor will hold water and are difficult to access) in distilled water several times to wash away all the milk. Dry with paper towels or napkins, then air-dry with a hair dryer on the no-heat (or low heat) setting. When it's mostly dry, rinse it with the alcohol. The idea is to displace any stubborn water stuck in nooks and crannies with the alcohol. The alcohol evaporates a lot quicker than water, so the end result is dry components without water causing galvanic corrosion before it evaporates. Be aware that alcohol will remove certain markings, dyes, and adhesives. So you may not want to pour it over everything, just places where you think there might be trapped water.
The keyboard is probably a loss unless you can get underneath the domes to rinse them out. In my experience they're extremely sensitive to contamination by dried gunk. Fortunately you have a fairly popular laptop model so getting replacement keyboard if needed shouldn't be too difficult.
After all this, set up all the pieces so they can get a lot of airflow, and put a fan next to them blowing air on them. If the area you live in is humid, you may want to put the pieces in an airtight box with a bunch of dessicant (stuff that absorbs water out of the air). Uncooked rice will kinda sorta work as a dessicant, but a real dessicant will work much better (easily obtainable from Amazon; retail stores like Walmart sometimes have it as DampRid).
You'll want to give it at least 3 days to dry. Preferably a week. Only then should you reassemble everything and hope (pray) that it worked and the computer will come back to life.
If the milk has already dried, you can still try all this but the chances of success are greatly reduced.
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