Most of them have "off the street" programs where you can get a guaranteed attempt at their assessment & selection course in your contract (others require you to have served for a few years first, and/or be a certain rank before you can try out). You still have to meet all of the necessary prerequisites & go through all of your initial training/screenings before you get to the selection course. Often times, but not always, assuming you didn't already get it in your contract, you get offered the chance to volunteer to go to selection at some point during your initial training.
Frequently (but not always), the contracts are set up in such a way that if you fail, whatever branch you're in gets to do whatever the fuck they want with you, which generally means getting stuck in a shitscrubber job (this is half the reason these contracts exist, since the overwhelming majority of people are going to fail selection–it's basically an open contract). If you fail, but not by quitting, they'll typically at least put you in infantry (depending on the branch) rather than making you a cook or whatever. If you fall out due to injury, you'll generally get the chance to try again.