To try to answer your specific concerns:
Basically the news is all good.
"One year ago ..
So, it is basically the norm to leave your first job after a year, assuming you're good. A year is forever in software. This is a non-issue.
Some more on that ..
Looking at the comments below. It would appear that amazingly ...
... the median tenure even at Apple is less than two years. Whoa!
(That would seem to be for all programmers, not just "newbies"; your first job is even shorter.)
This random statistic from the internet emphasizes that it's a non-issue if you have "only" stayed a complete year.
"different area ..
In software, every time you sit down to work, it's a different area. The very definition of being a skilled programmer is that you can instantly pick up and excel in any field.
I urge you to immediately get away from "A" to ANY field .. whether B, C or D or E.
Do not hesitate to take a contract with less $ if it gets you out of the "A" type-casting.
You don't want to be this guy: 1
"What would be the right moment to start looking...
"I couldn't afford spending several months searching..
With this point, you definitely face a serious problem, which you must fix if you have not already.
You can only get programming jobs from a position of strength.
To work in the dynamic, high-pay, high-risk field of software you MUST have a couple months reserve money on hand so that at any moment you can walk, and then leisurely look for your next contract.
If you have not achieved this "iron reserve," 2 do that FIRST and then enact your plan.
"Is it ok to honestly say during a job interview that I want to quit my current job because I always wanted to do B and have never been really interested in A? ...
It would be remarkably naive to actually say that:
It implies that you believe software is a "single field" skill. This is the one single thing you never, ever want to imply as a programmer.
The fact that you want another job is a non-issue. A sports team does not have to explain "why they want to win"; you don't have to explain "why you want to advance your career."
In general NEVER SAY ANYTHING NEGATIVE, IN ANY WAY, ABOUT ANYTHING. Your first job was "fantastic and challenging, but you're ready for the next challenge".
And note the critical point in a comment above: You'll need to have a good answer to the inevitable follow up "So why did you take a job in A?"
Fill your answer with overwhelming positives. "It was a great chance that came up straight away. Mr. Smith is super-nice and Ms. Jones is a fantastic Pascal programmer. Since high school I've been looking for challenges so I grabbed it.
For God's sake: never, ever, mention "money problems". Or indeed any problems.
To repeat it's totally OK and good to mention that you love the NEW field, but never knock the OLD field A in any way.
1 Brilliant actor Russell Johnson is often given as an example of a fantastic actor who's career was killed by being type-cast in one role.
2 Courtesy that important figure in Software, Napoleon Bonaparte.