German isn't the most widely spoken language and its total number of speakers may just be about to pass the 200 million mark. It is, however, the most commonly spoken language in the European Union.
German is part of the West Germanic branch of the Indo-European languages tree and is thus most closely related to English and Dutch than it is to any other language. English and German share many characteristics, particularly sentence structure and some grammatical points. This is not to say that the two languages don't have their differences as they do and here are a few of the most significant ones.
The alphabet is slightly different as German has its umlauted vowels (two dots above the vowel) which have a slightly different pronunciation and the double s sounding ß. The pattern of speech and letter pronunciation are also very similar to English and shouldn't prove to be too problematic.
Tenses are the biggest area of difficulty between the two languages as German features no continuous tense. This means that it can be difficult to express actions which are ongoing on in the present tense. Just think this one through carefully and conjugate the verb into the present tense. The past tense in German is most commonly the perfect past and can produce some strange translations. It's important to understand the past tense and it's best to revise how and when to use it.
Inflection is a strong characteristic of many languages and English is an exception. The meaning of a sentence in German can be altered by changing your tone of voice. Raising your pitch at the end of a sentence can change it from being a statement to a question. This can be a source of much confusion when starting to learn German and some German speakers may confuse English speakers whilst asking questions.
Word order is generally quite similar but there are a few exceptions. Verb and subject inversions in independent clauses, past participles at the end of a clause and the main verb forming the last word of a dependent clause are exceptions and can be quite confusing at first. Once you master this you will be able to reorder your sentences accordingly by instinct.
German nouns are all capitalized which is a really unusual trait in a language. Their language is also far more punctuated than English and this can lead to many red marks in any textbook during the early stages. Also, try not to get confused by cognates. If a word looks like an English one then it's generally best to look it up in a dictionary first rather than to just assume as this can lead to errors, sometimes embarrassing ones.
Even though German and English have many similarities it is still necessary to learn the key differences to avoid being caught out. Try to learn all new words and revise the use of tenses to ensure not to become easily confused. Firmly plant the sentence order exceptions into the back of your head and keep them in mind when ordering your sentences. Other than this, you should pick up German incredibly quickly.