Actually even though often used, "like" is less correct. "As" refers to the type of work, while "like" correctly refers to the quality of work. In this case if you say "he works like a guard", it is likely to refer to some other type of work that is somehow similar to the work an actual guard would do (similar hours/requirements but not the same job). You would also use "like" to mean that something that originally carries one function also happens to be able to carry another function. For example, "my grandmother is so strict that when we go shopping with together, she works like a guard against all potential suitors". Also, you can say "he works like an artist/slave" to mean someone works in a manner assumed to be common to artists/slaves, this meaning he works erratically or creativeli like an artist or very hard in bad conditions like a slave. For all those reasons, when you want to refer to the original intended purpose of a job, it is more correct to use "as".
Because the suffix of a verb changes depending on the noun and the tense. In present tense, if you are speaking about yourself, you say "Eu trabalho....". If you are talking to someone or about someone you say "Você trabalha.../Ela trabalha.../Ele trabalha.../O autor trabalha..." Have a look on google for a website called conjuga-me. It will show you all the different ways to write particular verbs, in different tenses and for different nouns.