How do I start trekking? First steps?

 There are many ways to start. Almost totally unprepared is one way and it actually often works better then when you buy loads of stuff and plan everything in advance. Just try it.
 
Physical preparation
That said, there are a few things that do come in handy if you want it to be a success. A little physical preparation really helps you to enjoy your trip. Don't you prefer watching the scenery instead of your next step and your watch? Don't you prefer inhaling fresh air with pleasure instead of breathing like an exited dog just to get sufficient oxygen? Do yourself a favour and go jogging, cycling or swimming or whatever sport you like for about 3 times a week, 2 months before you leave, in a gradually more intensive way. A start to run program is something my wife always uses before our trips, which starts really slowly and builds your condition up, step by step. I think the first day is just 3 minutes running and 3 minutes walking but after 2 months you can run for half an hour. Some discipline is needed, but it does pay back, guaranteed!
 
Mental preparation
The most underestimated and forgotten part is the mental preparation. That will be crucial not for starting maybe but for enjoying it and for wanting to do it again. Are you ready to face nature honestly? To go with what nature decides for you, like a thunderstorm? Is this trip a 'you against the elements' or 'you learning to know and live with the elements?' The more you are a control freak, which I also can be, the harder this is but the more you can truly learn from this experience! Also: are you ready to give up at least some of your usual cocoons and safe heavens which give you the false idea of being safe no matter what happens out there? Are you ready to give up some comforts like the idea of having a car ready to bring you everywhere, the comforts of a room for yourself to sleep, the habits in your daily routines of washing and eating? Are you also ready, mentally, to see the returns that you will get? Can you still notice and then enjoy breathing fresh and healthy mountain air? Can you feel that the human body was not made (over the past 4 million years) for sitting at a desk 9 to 5? It is basically made for walking and your eyes are made for looking at the horizon, not at a PC. Basically: are you ready to forget about 'society' and the normal that has been laid upon you by its rules, values and obligations, in order to rediscover the hidden but natural bond between you and nature?
 
Gear
As for practicalities: I don't believe you need a lot of gear to get started, although buying some lightweight materials do help when you trek for longer and longer periods and are especially unavoidable once you start trekking with a baby in your backpack. But that should NOT be your first trek! Basically, you just go out there and see what happens, but starting in the Ardennes rather then the Himalaya, taking some things like a rain jacket, enough water, good shoes, dry socks a map and a compass and doing a bit of training in the 2 months before leaving all help to make it a great experience. And please don't start with a 25km uphill daytrip in rain or you will only come home feeling like you never want to walk again!