The thing I want to know, I tell Sam Smith, is about the toilets.

You know, back in the day when you were working in a London bar and being sent there every 20 minutes to check they were OK. When you had to clean them, Sam, where you any good at it?

"I was awful," he admits. He is almost shouting. "And my bosses used to hate me because they knew it was just a stepping stone to get somewhere else. I always knew it wasn't where I would end up."

Right now, Sam Smith is in America. To be specific, he is in Boston. In a few hours, he will walk on stage for the first gig of his American tour, the latest step in what has been a vertiginous ascent. Back in the day for him was just four years ago, when as an 18-year-old he returned to London from school in Bishop's Stortford to work in bars, clean toilets and try to make it as a singer-songwriter. It's fair to say that, in 2014, he has achieved that last goal.

From guesting on tracks by Disclosure and Naughty Boy (La La La was the fifth best-selling single of last year), he has now racked up two No 1 singles of his own, Money On My Mind and Stay With Me (4.5 million sales of the latter alone), and won both the Brit Critic's Choice Award and the BBC's Sound of 2014 poll. He's released an album, In The Lonely Hour, another No 1 which has already racked up almost two million sales, and then there's the sold-out UK and American tours. Oh yes, and apparently he's "the male Adele" into the bargain. In the circumstances, vertiginous might not cover it.

"It's been very surreal," Smith admits. "There's learning curves throughout. From a year and a half ago I've been chucked into situations that I'm just not prepared for. And people expect the most of you as well. It can be really stressful at times. But I'm slowly but surely getting used to everything. The fame side of stuff I don't think I'll ever get used to. I kind of don't want to ever get used to it because the minute you do is when you lose yourself."

Smith, by the way, has reached the grand total of 22 years of age. He doesn't sound like he's lost much of anything yet. There's a sweet, youthful earnestness about him still. Perhaps it's no surprise that, at this age, he's not jaded yet. Then again, he started pursuing a music career when he was 13, had managers (a lot of them) from when he was in his early teens.

"I got involved in a few managements that would promise me the world. They'd promise me fame and money and everything. I'd be 15, and I would hear them and go back to my school friends and tell them 'oh my gosh, my managers told me I'm going to be working with this person next week. I'm going to be in the charts this time next month.' And then nothing would happen and I would look like a complete d***head in front of all my friends.

"You're going through a rollercoaster of emotion at the age of 16. I think you're going through enough, let alone all these grown men telling me what I should be doing and false promising and letting me down. Every single day I was let down."

Everything has changed now. What has he learnt about himself from being in the eye of the storm? "I've learnt that I don't necessarily want to be a celebrity. If you'd asked me two years ago, I was very attracted to the whole fame aspect. I would have said I wanted to be famous. I want to be photographed. But now there's nothing I want less. I want my music to be famous, not my face."

Of course he knows it doesn't work like that. He knows, too, that it is difficult to disentangle the music from the man. Like Adele, Smith has already shown himself adept at taking the building blocks of early 21st-century commercial pop and shaping them into a form of autobiography. In The Lonely Hour is a portrait of unrequited love (we are hopefully at the stage where the fact that said love was another man is hardly worth remarking on), a vision of one-night stands and aching regrets. He says now that writing it all down was a form of therapy.

"At the time it felt easy because I was going through so much," he recalls. But the ease didn't last. "The honesty on the record ... Sometimes it hurt me to listen back, especially during the making of it. I was thinking 'Should I be saying this stuff?' I was being so open. People ask me so many intimate questions in interviews, and I try to explain that you don't need to. Everything you need to know, the most intimate part of me, is in my music."

Anyway, he says, singing these songs live has transformed his relationship with them. "Having a thousand people sing back the lyrics 'I guess it's true/I'm not good at one-night stands' literally makes me want to break down every single time it happens. Because when I started writing this album, I was lonely and I felt alone in every aspect of my life. And now it's the complete opposite because of the fans and the response to the record."

Are men getting better at showing their emotions? "I think so. Because I've been surrounded by women my entire life, I've never thought about it. I never thought when I released this record that, for a guy, I was actually saying some pretty emotional stuff that normally guys wouldn't say. I think I'm always going to be in touch with my emotional side and be able to express that because it's just in my DNA. I think like a woman sometimes. I'm emotional and I get hurt and I'm okay to say it. I can cry and it's okay to OK and I think I'm always going to have that trick." That said, these days, he's not sitting in his room crying all that often: "It's all good." It might help, he adds, that he's currently dating.

I ask him about his earrings. In his videos you can see that he wears crosses in both ears. Is it a fashion statement or a statement of belief? "I went to Catholic school from the age of four. For me it is a statement. But it's a statement that I never talk about. It's my own personal relationship with religion. I believe in God and all of that. Yeah, there is a message behind it. I also have two tattoos on my fingers. There's a message behind that, but I'm going to keep that stuff to myself."

You're all the way over in Boston, Sam, I say. I can't see them. "There's a female and a male sign."

It's time to go. He has a gig to do. Yesterday he went to the gym for the first time. "Things have been very intense the past few weeks, so I'm trying to look after myself." He's excited about the tour. Singing live is now his favourite thing. "When I first started, I didn't enjoy it that much. I used to get very nervous and be very insecure on stage and not know myself. I found it very hard and I used to dread doing live shows. But now it's become truly the highlight of my life and I'm happiest when I'm on stage performing to people. It's the one thing I feel confident in, the one time and the one place where I truly know where I am and everything's fulfilled."

And he doesn't have to clean the toilets in between songs.

Sam Smith plays the 02 ABC, Glasgow, on Thursday, and returns to the O2 Academy on March 16 and 17. All concerts are currently sold out.