Just like a normal job there is an application process where you will have to be found suitable to be selected. Help yourself by researching the acio process on-line, be physically fit, smartly dressed and know the history of the regt you want to join.
In the mid-70s I walked into an ACIO mid-December with not a clue about what I was doing (except I really wanted to be a tank commander). I timed it cos I was hopeful I wouldn't be discovering the price of my actions over Christmas. As it happens I was in a couple of weeks after Christmas.
Having ended up in the RMP (don't ask) and realised I had screwed up, I got out in April, spent the summer on the dole and on the beach, then started again in June, this time demanding I be guaranteed a place in the RAC. There was some delay because RMP Records wouldn't let go, but I was re-enlisted by the end of August and a Catterick Commando by the end of September.
Course that was when the army was needing them, not feeding them.