Hello, I'm not sure I fully understand your question, because a text may sound professional in different ways depending on your professional area. Could you provide more information as to what particular use the text would have - a letter, e-mail, report? Nonetheless, if by professional you mean formal, my advice to you is to write it without grammar or orthographic mistakes, use accurate words and pronouns accordingly, with gender and number, and don't write with unnatural intimacy towards your reader. But, most important: write it naturally, give it a natural — not informal — tone to it. I believe it to be worse to read a flourished yet correct text than to read a fluid, natural and still formal text. If you're writing it in PT-PT some orthography might be different from PT-BR, some pronouns and structures should be more common such as the use of "vós". My last tip: check the guidelines of what you ahould write, models, formats etc. Hope this helps you. (:

Hi... I do not know if I understood your question very well but You must use a polite ... You should use a more formal language in the text, for example. "I would like to work with gardening" You should writing "I would like an opportunity in the gardening area" In portuguese it would be "Eu gostaria de trabalhar com jardinagem" You should writing "Eu gostaria de uma oportunidade na área de jardinagem" I hope I have helped you...

I answered in the comment section but just corrected it.