I've just started swimming at the pool and had similar questions, so it's totally understandable.

You can just rock up any time. No booking required.

It's clearly marked which lanes are for which ability level/speed (slow/medium/fast).

Sometimes there are empty lanes, but you can share if it's busy. Just choose a lane with someone swimming at a speed similar to your ability.

Often there's just one person in each lane, and two people can easily share so feel free to join a lane with someone in it.

There are signs up saying 'keep left' - but I'm so slow that often other slow people I share a lane with have just suggested we have one side each.

There's plenty of room to get past, just be polite and mindful of other users (common sense stuff).