The essence of professionalism is a duty of care to those you advise and to those you serve. It is your duty to advise them well so that they are protected from harm, whether that harm is to their financial status, their health, their reputation and standing. You also have a duty of care to yourself.
That implies knowledge on your part which you take steps to keep up to date, which is why professionals have to attend to their Continued Professional Development (CPD), and an obligation to do the best for the client whose interests you are protecting.
This in turn requires honesty on your part, including an ability to say you cannot help if you are at the limits of your ability.
You need to think about becoming “professional” once you have qualified. Those practising in the subject in which you have qualified will probably be Members or Fellows of an Institution, (e.g. Institution of Civil Engineers) or a Society (e.g. Royal Geographical Society) that is responsible for their standards and interface with society. These societies and institutions provide the means for their members and Fellows to become professional; they will require you to become Chartered.
Here is a down loadable PDF document that outlines what it means to be “professional”. Reading it you will realise that in many respects it is another form of “etiquette”, viz. that required to act as a professional. This document includes aspects of etiquettes described under the section “etiquette” on this website but goes beyond those. Here is the PDF document on professionalism.