When you look at a foreign language, you often find that it appears to be a complete mystery. Whether you are enchanted or just plain baffled by this, it is still none the clearer and anything other than English may seem just alien. Although both English and Spanish are both members of the Indo-European Language tree, they are from different branches. Whilst English is a Frisian language most similar to German, Spanish is a member of the Iberian romance languages and is most closely related to Portuguese. This means that there is more than likely to be some differences between the two languages, making it a little more difficult to learn.
At a very basic level, there are even differences in the alphabet and pronunciation. The Spanish alphabet has slightly more letters including ñ which is pronounced like the N in onion and the ll which sounds more like the English double E sound. The letter R is rolled and double R is doubly rolled in Spanish words by vibrating the tongue on the upper teeth. The letter J and occasionally the letter G are both pronounced more like an emphatic H as in Jalapeño. Other than this the pronunciation shouldn't raise too many difficulties. You also have to remember to put question and exclamation marks upside down at the start of a phrase as well as at the end.
The two languages also have a contrasting meter. This is to say that the pattern of stressed syllables is different. In English we tend to stress the ante penultimate syllable whereas the Spanish only stress a syllable when there is an accent present. This can be quite confusing for English speakers.
Tenses can also be a bit of a bother for many language learners and there is not always the predictable correspondence of tenses between two languages that you might expect. Spanish uses different forms of the past tense in different situations and to refer to continuous and completed actions in a different way to English. Always revise the uses of tenses as well as how to form them. This will help make your Spanish far more accurate and will avoid confusion when trying to make arrangements.
Latin has given both English and Spanish a large amount of their vocabulary lists and it is therefore easy to see a few similar looking and sounding words. This can make things really quite easy when the words do in fact mean exactly the same thing but it can be a real nightmare if they have different meanings. This can lead to a slight amount of embarrassment if you get unlucky but you can generally protect yourself by looking up new words and remembering their spelling.
Spanish and English are bound to be different; if they were the same then we would all be fluent and where's the fun in that? They belong to different parts of the language tree but we can still adapt fairly quickly. As long as you get into the mood and try to be authentic then you will generally have no problems at all.