I'm thinking of quitting my job after a month. How would I explain this to future employers?

As long as you don't have a pattern of repeatedly having short jobs (and especially if you leave them for shady reasons), you don't have that much to fear from quitting after just one month.

This job is even so short that you can pitch the short duration as a positive thing. It took you only 1 month to realize that this job is not something you want to do long-term, where others might take half a year or longer to come to the same realization.

If the 1-month job is not your current job but one further in the past, then you can consider leaving it off your resume and merge it with the job-searching gap you already have in front or behind it.

I would suggest you leave this job entirely off your resume. Mentioning it cannot help you and may harm you.

You don't. A 1 month engagement adds no value to a resume. It's just a continuing gap in your previous unemployment. Resumes should include only relevant positions that demonstrate your ability to do a job. You wouldn't list a job from 12 years ago in a different field of work for the same reason.

Find a new job before you quit.

Having a job will make it easier to find the next one, even if it is in a completely unrelated field.

I don't think anyone would see a short employment as a red flag. Especially since you have an easily understandable explanation: "I realised I don't want a career in sales". What might be a red flag is if you are unemployed for 6+ months.

Depending on your contract, or other agreements if you have not signed one yet, you might still be in a “provisional period”. Sometimes these are short, but in some jobs three months is not unusual.

If you are in such a period, remember that it works both ways: as well as the employer being able to let you go for any reason, you can let them go too and it isn't really quitting - you have just decided, quite late in the extended interview process, not to take their employment offer over a longer term.

As others have said, it is generally better to have something line up before you leave if that is practical, so if you do have a month or two left of a provisional period perhaps delay your decision a little longer. This gives you time for either finding the next step, or to change your mind - take the flexibility this gives you to take your time making sure you do the best thing for yourself.

On the question of how it will look: if you say to an interviewer that you are in a provisional job right now, but you are thinking that it is not the right place for you, most will understand. Be prepared to be asked why you think their job will work out better and have a good answer ready. If you do put the current job on your CV, I would mark it as “provisional period, ongoing” for clarity, this tells someone looking at it that you are not only good enough to be employed (because you are currently employed, ish), but you are still also available almost immediately rather than needing to work out a long notice period (while not usually a priority consideration, this could be the deciding factor between two otherwise identical candidates).

In many locations, you have a probation period of three months for example, which means that for three months the company can lay you off with no reason, and you can quit with no reason.

The intention is that both sides have more time than just the job interview that the employee is happy with the job, and the company is happy with the employee, and both sides want a long term employment relationship. Quitting or being laid off within one month is absolutely normal and no reasonable employer would hold it against you. It’s enough to say “I assumed the job would be to do A, B and C, but they wanted me to do X, Y and Z”. Obviously better for both sides to quit.