There are many words that are very similar between English and Spanish. These words are referred to as cognates. The majority of these cognates are Latin-based. The Spanish version came straight from Latin, and English acquired many Latin based words from French. Cognates can be very useful for a native English speaker, because after learning a few rules for converting an English word to its Spanish cognate, you can effectively increase your vocabulary by thousands of words. It's no magic pill to instantly speak Spanish like a native, but it will give you an extra boost.
One important note. Your pronunciation must be up to speed, otherwise Spanish speakers will have a hard time understanding what you say, even though the words are very similar. Above all, make sure you pronounce the five vowels correctly.
False cognates or false friends are words that look extremely similar in English and in Spanish, and may even come from the same Latin root, but do not have the same meaning. You need to watch out for these stumbling blocks to avoid misunderstandings with native Spanish speakers.