Begin with nouns (os substantivos). There are two genders of noun in Portuguese and many of these have the familiar -o and -a endings of Spanish nouns. Additionally, many nouns form their plural simply by adding -s. However, according to Orbis Latinus, there are many exceptions to these rules, including how to derive feminine versions of masculine nouns. Orbis Latinus can help you learn exceptions as well as how to make diminutives and augmentatives of nouns (see Resources).
Continue to adjectives (os adjectivos). These generally follow their noun and have many different forms. For instance, branco (white) shows gender and number (branco/branca/brancos/brancas), while others show only number (feliz/feliz/felizes/felizes). To learn more about Portuguese adjectives, see Resources.
Learn personal and other pronouns (os pronomes). Although Portuguese does not have a case system, it has retained declension of pronouns to show how their function within a phrase. With personal pronouns, Portuguese shows the following forms: subject, object, indirect object (with and without a preposition) and a special "with" form. For the first-person singular pronoun ("I"), the forms are: eu, me, me, a mim, migo/comigo. For the declension of other pronouns and words like minho (mine), see Resources.
Turn now to verbs. This will likely be the most difficult part of your study because of the sheer number of forms of verbs in Portuguese. Begin by learning that there are three different types of ending in the infinitive (to) form: -ar, -er and -ir. Examples include falar (to speak), dever (to have to) and pedir (to ask). Portuguese has three persons (first, second and third) in two numbers (singular and plural).
Learn the simple tenses of Portuguese verbs; these are the forms of verbs that communicate their information using only one word (such as, amo, "I love"). Pay special attention to the personal infinitive, a form of infinitive that shows a relationship to person and number; there are both simple and compound forms of this verb, and this is a feature not found in many other Romance languages. Portuguese has eleven simple tenses; see Resources.
Turn to the compound tenses, verbs that communicate their information with the aid of a helping verb. For instance: terei amado ("I will have loved") and teria amado ("I would have loved"). Portuguese has eight compound verb tenses: see Resources. Also learn the gerunds and participles of Portuguese verbs (amando, "loving" and amado, "loved"), found under "Portuguese Verbs" in Resources.
Learn Portuguese syntax. While most learners of a language will develop a feeling for syntax as they continue their study, students who wish to have a broader depth of grammatical knowledge may want to make a more serious study of syntax. See Resources for an in depth article on Portuguese syntax.