In learning Spanish (or any language for that matter), beginners tend to make some unnecessary mistakes. Of course, it is natural to make mistakes because that is part of the learning process.
What is embarrassing is that committing them makes one sound less intelligent that one actually is. However, some of them can be avoided. The following are some mistakes that can be sidestepped.
Cognates
These are words that have the same form in both English and Spanish. Since both English and Spanish share many words derived from their Latin roots, it is not uncommon that these look-alike words have the same meaning.
Or not.
There are so many exceptions to this fact, and it should be a good idea to study a list of these false cognates and partial cognates. EMBARAZADA usually means pregnant rather than the sound-alike embarrassed. The Spanish VIOLADOR usually means a rapist, and not just somebody who violates some traffic rules (violator).
Pronouns
In English, a sentence (with few exceptions) usually needs a subject.
In Spanish, that would not be altogether true. Where it can be understood by way of the context, the subject of a sentence (which is a pronoun in English) can be omitted. Usually, it IS omitted.
Normally, it is not ungrammatical to include the pronoun. Using it would sometimes make the sentence clunky, and invites attention.
Prepositions
This is tricky because prepositions in Spanish are challenging. Learning the purpose of prepositions is helpful, rather than banking solely on their translations. This can help in avoiding mistakes such as PIENSO ACERCA DE TI in place of PIENSO EN TI (I am thinking of you).
Word orders
Except for placing adjectives after the noun they modify, it is not really bad to follow the English word order.
As you progress in your Spanish lessons, pay attention, however, to the many times where the subject is placed after the verb. You will notice changing the word order do change, however subtly, the meaning of a sentence.
This is the same case when some sentence construction in English where a preposition is sometimes placed at the end of a sentence. This should NOT be imitated in Spanish.
Idioms
All languages have their own idioms which sound perfect in their linguistic contexts. There is great danger in having these idioms translated into another language, whether from English to Spanish or vice-versa.
An example would be EN EL ACTO which means on the spot. The word for word translation in Spanish would end up like EN EL SITIO and in the act in English, which are both wrong.
Articles
Learn to use the commonly-needed articles (UN, UNA, EL, LA, LOS, LAS) in your sentences. Incorrectly using them will not keep you from being understood, but you will be marked as an awkward speaker.
(If its any consolation, this is the same problem of non-English speakers trying to use the articles A, AN and THE in their English sentences.)
Others
There are other mistakes that English speakers have to be aware of (and avoid, if possible) in polishing their Spanish. This would include not learning to use the subjunctive mood and ignoring proper pronunciation.
The biggest mistake, of course, is being afraid of making mistakes. They are part of the learning process, learning Spanish included. Spanish people will appreciate your attempts to speak correctly, and they dont usually remember your mistakes.