It varies from train to train but typically there is a hook to hang your bike from, and maybe a slot to put your back wheel into. Sometimes there are straps to secure your bike, sometimes not, so I always take a couple of bungee cords to use. If you don't secure the bike, it tends to swing about and I don't like that. Some trains have little compartments to wheel your bike into.
If you are worried about other bikes swinging into yours, you could always take some bubblewrap to put between them.
I've had a bike reservation before now but got on my train and found that all the spaces taken up by unreserved bikes! If you have some bungee cords, you can secure the bike somewhere else and if the train manager (or whatever they call themselves these days) objects, then show them your reservation and ask them to sort out the owners of the other bikes.
If the train is crowded, make your way to your bike well before the station you intend to get off at. I got trapped at the wrong end of a packed train once and had to run down the platform at my station, hop back on and just got my bike in time. You don't need stress like that.
If you mean secure as in 'do they get stolen' - I imagine that it is theoretically possible. You could always take a cable lock and at least lock the wheels, even if you couldn't find anything to lock it to. I prefer to sit near the bike compartment so I can keep an eye on my bike. You may be able to book a seat nearby, and if not you can often find another seat and ignore your reservation. So far, nobody has forced me to sit in the seat I reserved.