The 23-year-old has been named in Roy Hodgson's squad to face the Republic of Ireland and Brazil later this month and admits he feels "overwhelmed" by the honour.
"I'm over the moon," he told Sky Sports. "Obviously, growing up as a kid you dream of playing for your country. I'm thrilled to get that chance. I'm overwhelmed.
"My phone didn't stop yesterday. It was a nightmare with phone calls and text messages. Everyone is really happy for me and I can't wait for next week.
"I had a couple of holidays booked and they're going to have to wait now. But obviously I'd much rather be going away with England. I've always got time after.
"I've been to Wembley before a couple of years ago with the national team but to be there with the national team is something else.
"I know a few of the lads from the Under 21s like Daniel Sturridge, Tom Cleverley, Danny Welbeck, Phil Jones and Kyle Walker but the rest will be new. It helps having those lads there."
McCarthy has impressed since returning to the team last month after damaging his shoulder in November and while he came back too late to save the Royals he is optimistic for next season.
"I was in the team and got that freak injury at the end of the QPR game," he added. "Originally they said I'd be out for the rest of the season but I rehabbed really well and came back early.
"Everyone's disappointed with how the season has gone with us going down. But we'll obviously look to bounce straight back up.
"We've shown in the last few games we are capable of beating teams and playing good football so the mood in the camp is still really good."