Gregor's attitude and his reaction are all rooted in his insecurity about his job. Because he is the sole breadwinner for the family, and he is working to pay off his parents' debts to the head of the company for which he works, the pressure is on him to perform and support his parents and sister. Thus, when he wakes up and realizes something is wrong, his first thoughts are that he is late to work, and that he must get out of bed as soon as possible.
The next train left at seven o’clock. To catch that one, he would have to go in a mad rush. The sample collection was not packed up yet, and he really did not feel particularly fresh and active. And even if he caught the train, there was no avoiding a blow-up with the boss, because the firm’s errand boy would have waited for the five o’clock train and reported the news of his absence long ago. He was the boss’s minion, without backbone and intelligence. Well then, what if he reported in sick? But that would be extremely embarrassing and suspicious, because during his five years’ service Gregor had not been sick even once. The boss would certainly come with the doctor from the health insurance company and would reproach his parents for their lazy son and cut short all objections with the insurance doctor’s comments; for him everyone was completely healthy but really lazy about work.
So there it is. Not one mention of how to actually go about doing these things, and seemingly no reaction to the fact that he is now a giant bug. Instead, his first concerns are how to get to work, or failing that, how to postpone returning to work long enough to escape his current situation but still keep his job. This is where the humor lies, as the issues of the novel are revealed. Anyone else, upon waking and discovering themselves a "monstrous vermin", would most likely panic and struggle to determine why they changed, and how to change back. But the pressures of work, and his insecurity in his family cause Gregor to first question how this situation will affect his job.