Lake Como

I would definitely arrive at least a few days earlier to give yourself a buffer should something go wrong with your flight. Also, It gives you time to get over jet lag and adjust to the time zone. Have you been to Italy before? Varenna (Lake Como) is a very quick, easy train ride from Milan. There is certainly plenty to do in and around Milan. However, usually, for a first trip to Italy, I might suggest seeing Rome, Venice and/or Florence. You could fly into Venice for a few days and then choose a couple other cities and work yourself to Milan/Varenna. Probably easiest would be Bologna/Modena/Parma after Venice.

Due to the time change and being jet-lagged, I like to spend at least the first 3 nights of my trip in the same hotel and city I arrived in to get acclimated. Depending upon when and from where you fly in, that would give you some full days in Milan to get your bearings. I would not miss iconic landmarks, such as climbing up the Duomo, seeing The Last Super (make reservations in advance), Sforza Castle, La Scala Opera House, and the Galleria Vittorio Emmanuelle for starters. It would give you time to leisurely go to the Central train station before the day of your trip to Lake Como to purchase train tickets in advance without same day time crunches and stress. This way, if you leave in the morning, you will be well-rested and more familiar with your surroundings. That is how I like to operate in a new city.

A lot depends on when your plane arrives in Milan. If evening, there aren't many trains left on the schedule for Varenna. As it's a regional train, without seat reservations, you could end up standing. Also, the schedule is a bit of a suggestion, so that's too much for a first day.
Hotel Berna is a great first night (or last night) hotel, near the central station. Then take an early train to Varenna the next day.
Do you alright have accommodations? If not, reserve something asap. This is very late for summer in Lake Como,
What else are you doing in Italy? Switzerland? How much time do you have? Interests ? A few more details would help a lot.
Have a great trip!

If you must be at Lake Como on a specific day for a special event you can't miss (like the start of a tour or a wedding), then I would go at least two days earlier, to account for possible flight delays/cancellations, and to get used to the time zone.

Personally I can get used to the time zone very quickly, but my rule of thumb is that you need approximately 1 night for every 2-3 hours of time difference. So for example if the time difference between your home and Italy is 6 hours, you should expect to need 2 to 3 nights to get used to the time. In my case coming from California (- 9 hours difference from Italy, like for you in Oregon), I need 3 or 4 nights to be fully adjusted. However I really force myself, by immediately adapting to the new time, therefore I have meals according to the new time and make sure I have the window curtains open in the morning so that I wake up with the sunlight. The sunlight and the meals are what regulates our internal biological clock.

However if you don't really have a very important event to attend, you can arrive the day before. Just know that you will be a bit jetlagged so when you travel east, you will tend to have a hard time getting up early in the morning for the first few nights.

You don't need to tour Milan for a few days first, unless you want to. You can go straight to lake Como and acclimate yourself there, which is a pleasant place to be.

The closest international airport to lake Como is Milan LINATE (LIN) and, a little farther, Milan MALPENSA (MXP). The former handles only European flights, but if your first airport from Oregon is any hub in Europe, you can select LINATE as your final destination.

If you want to keep it as a simple first trip, yes fly in to Milan. Then perhaps two nights there and then easy train ride to Lake Como. You can enjoy being there and taking a boat from Varenna to other locations on Como. I'm sure it will depend on where you are being invited to! Enjoy! It will be lovely to have a few days there!