An accent is used over vowels to indicate a stressed syllable or the fusion of vowels. Learning where and when to place accents on words is just part of the learning process but I’d like to map it out for you in case you have any doubts. The section on the trema (¨) has a lighter font because due to the Spelling Reform, the trema is no longer used on words of Portuguese origin (only on foreign words now).

Agudo (´)
It’s use is conditioned by the rules of graphic accentuation, including stressed and open vowels such as a, e, and o or above the stressed vowels i and u.

Ex. Macapá (capital of Amapá state), médico (doctor), tórax (thorax), vovó (grandmother), língua (language/tongue), múltiplo (multiple).

Circunflexo* (ˆ)
Used above stressed and closed vowels such as a, e and o, in accordance with the rules of graphic accentuation.

Ex. lâmpada (lamp), você (you), ônibus (bus), vovô (grandfather).

Grave (`)
Indicates a crasis (contraction of a vowel or diphthong), such as the fusion of two a‘s (in general, a preposition and the article).

Ex. Fui à festa sem ser convidado (I went to the party without being invited).

Til (˜)
Used above a and e to indicate nasality.

Ex. pão (bread), mamões (papayas), ímã (magnet). 

Cedilha (ç)
Used only with the letter c when followed by a, o and u, to indicate the sound of the phoneme.

Ex. maçã (apple), estação (station, season), espaço (space), açúcar (sugar).

Apóstrofo (‘)
Used to indicate the supression of a phoneme in a word, in order to avoid repetition or cacophony (harshness in sound).

Ex. d’água (of water).

* – The circunflexo is also called “chapeuzinho” (little hat) informally.