"The last eight months have been eight months of joy with what has happened with my football, but I'm definitely not going to get ahead of myself," he said on Wednesday.
"So many people have supported me family, friends.
"I'm very grateful and I just hope I can be a role model for other young African people to follow their dreams and never stop believing that they can achieve them."
Kosmina said Kamara's acquisition was a fantastic story.
As a six-year-old he fled Liberia with his family, coming to Australia as a refugee.
From kicking a ball around in the park with his brothers he first played club football with the Croydon Kings under 8s before moving on to Salisbury United and the South Australian Sports Institute.
He also spent time with SANFL club North Adelaide's U13 development squad before abandoning thoughts of a career in Australian rules.
"I saw him come on the other night and the way the crowd reacted to him was brilliant," Kosmina said.
"I asked some people at Croydon about him and they said he was not only a good quality player but also a good quality person.
"For a 15-year-old he's quite mature, physically he's a fantastic specimen and last Friday night and this week at training he's looked comfortable.
"He's a natural."
Kamara was the youngest member of the Australia's FIFA under 17 World Cup squad in Mexico and has played all but one of united's National Youth League games this season.
He has been included in Adelaide's travelling squad for the match against Sydney FC on Thursday.
- AAP
