“C’était la première chose qui m’est venue en tête”. Or you could also say it like “C’était la première chose qui m’est venue à l’esprit”.

So ‘Something’ means ‘quelque chose’ in French. ‘Thing’ means ‘chose’ in French as well. In French just like in English, you would not want to use ‘thing’ to describe a specific object or action.

C’est la première chose qui me soit venue à l’esprit.

Strictly speaking, I believe that the presence of the phrase « la première » requires the use of the past subjunctive « soit venue » instead of « est venue ». Then again, both might work.

Also, I would personally use the present tense « est » instead of « était », since the past tense is used in the part « soit venue ». I’d rather avoid using the past tense twice.

Apart from the debatable question of which tense is to be used, I’m a bit surprised to see all answers propose this kind of translation:

C'[est|était] la première chose qui me [soit|est|était] venue à l’esprit.

Maybe (as a French native speaker) I have a bad comprehension of the implicit sense the original sentence may bring in English, but the first translation that came into my mind (oh! What a coincidence) is rather:

C'[est|était] la première chose qui [vienne|vient|venait] à l’esprit.

In other words the original sentence might talk about what comes to the mind of anybody, rather than to the speaker itself.