OK, here goes... My son has been arrested 3 times, and I went with him to the police station the first time (but not the second, because he had assaulted me, and not the most recent time - this Saturday night - because now he's 17 he doesn't need a parent with him apparently, and didn't want me there).
I must say, I was pretty traumatised that first time, so you have my sympathy.
You say "what will happen later?" so I guess either he has an appointment to present himself at the police station to be arrested, or they already have him in custody and you are going to go down there. Like West says, he has to have an adult present, but it doesn't have to be you. If your relationship with him is good and you want to support him, I'd say be brave and go. If you think he should face this without parental back-up, you can refuse, and they will find a social worker or other responsible adult to sit in on the interview. If you are very upset, it might be best if it isn't you, because the responsible adult's role is to make sure your son understands everything that is being said to him.
What happens exactly will depend on your son's attitude and whether he admits to what they believe he's done. They will treat him decently in any case, but if he behaves rudely or stupidly or is a bit of a d*ck, they may 'forget' certain things (like breakfast for several hours).
If he has been arrested and is in custody already, he will be being held in cells until you/a responsible adult arrives. He will have had his possessions temporarily confiscated, plus anything he might be able to harm himself with - including for example the string in his tracksuit bottoms, if he's wearing any. He will have been given 'disposable clothes' to wear if they take any of his, e.g. for forensic analysis. If he has an appointment, he won't have to spend time in the cells - he'll just go straight to interview.
There will be paperwork: basic details like name, address, parents, etc.
He will have fingerprints and a DNA sample taken. This can be a strand of hair or a mouth swab. These records are kept forever.
He should be offered a lawyer, or you can arrange one for him. Burglary is serious, so he should probably have one. He should definitely have one if it looks like he is being charged with burglary and says he hasn't done it.
He will be interviewed. The interview will be recorded. He'll be asked questions about what's happened. Telling the truth is very definitely the best plan. The responsible adult will make sure your son understands. The lawyer will advise him on what to say.
If your son has done what he is charged with and admits it and co-operates, the whole process will be shorter and easier, and the police will probably be quite pleasant/friendly. They have more experience than you do of young men/boys doing stupid things and they will be quite used to this kind of situation.
The outcome will be different, too. If he admits to what they think he has done, and expresses regret, and (like you say) hasn't been in trouble before, he will probably get what is called a 'reprimand'. This is a formal warning. Nothing happens after this. It is removed from his record when he turns 18, and does not need to be declared when he applies for a job, tho' it will show up if he has a CRB/police check. If the offence is more serious, he might get a 'final warning', which is like the reprimand, but also means he is referred to the Youth Offending Team afterwards. I can tell you more about this later, if that is what happens to your son.
If he doesn't admit or hasn't done what they think he's done, I think he will then have to go to court, and I think he will be bailed in the meantime... But I can't tell you about this, because my son has always admitted what he's done, and hasn't been to court yet.
If things are as you describe, it is not at all likely that your son will be facing prison/young offenders institution. But if the offence is more serious than you know (for example, not just theft of a lawnmower, but also violence), I guess it's a possibility. Then he would need to go to court. But as I say, that doesn't sound like it will happen.
Throughout, whatever happens, the police will probably be very pleasant to you. They will probably blame you less than you blame yourself. Good luck. :)