There are a lot of different ways but one very basic method is simply to think of: nothing. This is much harder than it sounds (or maybe it does sound hard). But basically it's very easy for your mind to just go off on some random train of thought without even realizing. This form of meditation conceives your mind as a train station with trains pulling in & out. Most of the time we don't even think about what train we board and we're halfway across the country before we realize we even boarded it. This meditation tries to teach you to recognize you're about to board the train so you can decide whether you want to board it.

In practice it is too hard off the bat to think of literally nothing so this is why people focus on their breath.

A good amount of time to meditate to start is 10-15 minutes. Ideally you would do it every day but it is difficult to be consistent. Like surprisingly difficult to take 15 minutes out of your day to do literally nothing. Something in the mind resists it (probably the bad part).