The winger is one of three new caps in the Scotland XV to face Argentina in Rosario.
"I'm over the moon," he told The Herald. "I felt I have had a good run of games, especially the last five outings with the Warriors games they were all won and I cannot wait for this opportunity. It has been frustrating having to wait for the chance but I will give it my best shot.
"I felt I had a really good game away for the A team in Italy earlier in the year and then broke my foot in the last second of play. But I believe everything happens for a reason and I went away from that and trained as hard as I could. I now feel it is my time and I have never felt in better shape."
Thom added: "I have had a good look at Argentina, I have not seen the wings that much but have had a good look at Tomas De Vedia on video playing for London Irish.
"Of course I am very aware of what Felipe Contepomi can do after playing against Leinster, and you don't have to tell me anything about my Glasgow mate Bernie Stortoni.
"He is great player at full back and I am fully clued up on what he can do. I have to take it like every other game and play as I know I can.
"The Argentina defence sometimes gets tight and I have worked with Dan Parks on those crossfield kicks, so that might be an option. I will just aim to make myself available as much as possible.
"When I came to Scotland I set myself the goal of possibly breaking into the Scotland squad inside two years, and I have achieved that.
"If only my grandad Fred Thom was still alive it would be an even bigger dream come true. He is the one who got me into rugby and because he was from Glasgow, he gave me the Scottish blood. I was at an international school in Portugal where rugby was not played and he was the main influence in getting me into school in England so that he could watch his sons playing. I will be thinking of him.
"On a personal level it is massive. It will not hit me until I step on to the pitch but I'm equally sure I will love every minute of it."