처음으로 시험을 본거 치곤 잘 했다 = 처음으로 시험을 본 것 치고는 잘했다. => (I/You/They) have done well considering that it was the first exam (I/you/they) have taken. => (I) have done well if I take into account that it was the first exam I've ever taken. 치다(v) here means: take/consider into account (in judgment, calculation, etc). So -ㄴ 것 치고는 means "considering that ...". Examples: - 겨울날(인 것) 치고는 별로 안 춥다 = 겨울날인 걸 생각하면 [감안하면] 별로 안 춥다 (more formal) = It's not that cold for a winter day. - 처음으로 시험을 본 것 치고는 잘했다 = 처음(으로) 본 시험 치고는 잘했다 (more colloquial).

시험 pairs with 본 거. 시험 보다 means taking an exam. Since 치고는 follows a noun you need to make that into a noun clause hence 본 거. 치곤 close in meaning as considering. You use it to emphasize something as exceptional if you use the previous subject as a measuring standard. So in your case, taking an exam as first time as the base fact to measure against you have done well in an exemplary manner.