The phrase "at first decided" is correct and usable in written English.
It can be used when describing an initial choice or conclusion that was made before further consideration or change.
Example: "At first decided to take the job, she later reconsidered her options."
Alternatives: "initially chose" or "originally resolved".
As the hackers escalated their attack, the studio at first decided to pull "The Interview" from distribution.
The Obama administration at first decided not to take the case to the Supreme Court and agreed with the A.C.L.U. to release the photos this month.
This is why Google at first decided to censor its Chinese search service: there was no other way to be widely accessible in the country.But China is by no means the only country erecting borders in cyberspace.
When Joseph P. Dowd, a small-time lawyer, met Mary Corcoran, newly widowed, at the bar of Harpoon Louie's in Winthrop Harbor, Ill., in the spring of 1999, he at first decided not to approach her.
The authority agreed to restrict the E-ZPass tags and the rail and bus passes just a day after the board, meeting in a session closed to the public, had at first decided to go to court to keep them.
When people who had seen this early version of the film offered divergent reactions to the message -- especially foreign journalists, to whom the conflicts in Rwanda and the former Yugoslavia had a somewhat different meaning -- it was at first decided to trim out Rwanda, Bosnia and Kosovo but to keep the connection to Sept. 11.
Budgets are first decided at any company, and only then is pay determined.
Are you sure you didn't look at the butterflies' eggs first, decide what they looked like, then put the "fruit" bowl together?
Have you ever gotten into trouble for the way you jump out at people when you first decide to photograph them?
At first, he decided to forgo college, running for a bookie at a local racetrack.
At first I decided I just wouldn't get involved in their problems anymore.
