How do I start meditating as a complete beginner?

Sit somewhere you can be still. A chair with your feet flat and your hips slightly above your knees works as well as a cushion, and for a lot of bodies it works better. Pick one thing to put your attention on and stay with it for the whole session, usually the breath at the nostrils or the movement in your belly. Then expect to lose it constantly. Noticing that you have gone and coming back is the exercise rather than an interruption of it, so a session with fifty departures is fifty repetitions. Start at a length you will genuinely repeat tomorrow, and write two lines afterwards about what your attention actually did, because meditation gives you almost no signal from inside a session.