There are lots of amusing posts on this site about how cold the winter here is, how you freeze after just a few minutes outdoors. Now, you don't have to be cold in the Finnish winter if you only dress right!
The most important thing is to wear long underwear. Don't come tell me you are cold if you are not wearing long underwear! The long underwear should be 100% cotton. Stockmann department store stocks these. Technical fabrics are fine if you are trekking out in the wild for days on end, but in an urban setting, where you will be popping in and out, pure cotton is more comfortable indoors.
Dress in layers. While it is cold outdoors, it is warm indoors, a lot warmer than in so-called warm countries in the winter. So when you come in, you take off the outer layers (overcoat and woolly sweater). Be sure to take off enough clothes so as not to perspire. If you are wet, you will be very cold once you go out again.
Pants are a lot better than skirts. The pants should contain at least some wool, just plain jeans are too cold. This is especially important if you are wearing a short coat. If you absolutely want to wear a skirt, you should wear tights too and something called villahousut on top of your tights and under your skirt. Villahousut are knitted woolly shorts that will keep your lady parts warm.
It is crucial to cover your head. Yes, you will have hat hair. In only mildly cold weather you can make do with thermal ear muffs. Make sure they are the kind with fur also on the inside, next to your ear, not just on the outside. You can find these in the market square.
It is also crucial to wear a woolly scarf around your neck and something on your hands. Start out with gloves in the fall, then move on to gloves with lining or wool mittens and finally leather mittens with fur lining.
You need at least two sets of winter boots. The first you wear when it is raining or the streets are full of soggy snow. The second you wear when it is very cold and dry. It is best to have two pairs of both kinds, so that you can alternate between them giving the boots a chance to dry out. The boots for very cold weather must have fur or wool lining. The problem with warm boots is that they are hot to wear indoors. I have found Gore-Tex to be a good material for countering this problem. The soles of the boots must have a non-slip profile.
When visiting someone, take off your boots and change into slippers. On casual visits, especially among the working class and agrarian population, it is okay to move around in your socks. For formal events, you must bring indoor shoes.