"I'm 26 now and this is the time (in the Premier League) for me and I'll be doing my utmost to make sure it works out," said the Blues new boy.
"I know I'll have a fight on my hands - but people said I wouldn't cut it in the Championship and some said I wouldn't even make it as a professional. I proved them wrong and I'm happy to keep doing it if I have to. It won't be easy but this is the challenge I wanted.
"It was a big thing for me and I wanted to be sure this was right but in the end I just wanted to play Premier League football. Everyone has their opinions and I'm sure there are some people who reckon I won't cut it at that level.
''But the fact is you will never know if you don't take your opportunity."
Johnson has also revealed that he put a call into his pal and ex-Cardiff colleague Michael Chopra, a known top-flight flop, before agreeing to join Alex McLeish's troops to discuss the perils of playing in the Premier League with the striker.
"I spoke to Chops about the deal and he just said to me I had to go for it,'' he told the Birmingham Mail.
''It was difficult for him up there (at Sunderland) and, for whatever reason, it didn't work out for him the way he would have liked. But he had that experience of playing in the Premier and that's something that you strive for.
"He told me I had to take the chance and I think that's the mentality of most of the lads."