The first thing to do is to spot the verb of the sentence. Each independent clause has only one main verb, which may be accompanied by an auxiliary or "helping" verb. Two or more main verbs give you a compound sentence. Verbs are words describing action or existence or -- in simpler words -- what a person or thing "does" or "is." Spotting more than one verb must also prompt you to identify the sentence's conjunctions: words that join together words, phrases or clauses (for instance "and" or "but").
Nouns are words describing a person (John), thing (knife), place (Montana), quality (sadness) or act (graduation). In a sentence, nouns are used to describe who does an action (subject) and who receives an action (object). In "John drove the car," "John" is the subject, "car" is the object and both are nouns. In case speakers want to avoid repeating obvious nouns ("John is great. John gave me a gift"), they can replace them with pronouns, such as he, she, it, they ("John is great. He gave me a gift").
Adjectives are words describing specific qualities of a noun. Therefore, whenever you find a word saying for example how tall a person is or how pleasant a view is, you have an adjective. On the other hand, adverbs' role is to describe verbs ("John carefully peeled...") and even adjectives themselves ("He is incredibly rich"). You can identify most adverbs by their characteristic "-ly" ending. Conjunctive adverbs (such as also, hence, furthermore, nonetheless) join two clauses together. Since they are not strong enough to link two independent clauses, you can find them after the semicolon dividing the two clauses ("...blow; hence, the balloon...").
Prepositions are words describing the connection between a noun and another noun, verb or adverb. For example, in "The book is under the table," "under" shows the relationship between the book and the table. Interjections are outcries, words that you use in a sentence to convey emotion. They may be meaningful ("Good grief, I forgot!"), or even just meaningless exclamations ("Wow"). Interjections are not part of formal speech and usually -- but not always -- you find them in sentences ending with an exclamation mark.