The letters and sequence of the German alphabet are nearly identical to the English.  However, German has four letters that English-speakers might find unfamiliar.  Three of these are the umlauted vowels:

Ä / ä, Ö / ö, Ü / ü — Besides transforming a vowel's sound into something wholly un-English, the two dots also change the meanings of words otherwise spelled identically.  It is thus risky to ignore or omit this distinction.  On keyboards not featuring umlauts, these letters can be conveniently represented as ae, oe, and ue, respectively. Thus the word schön (meaning "pretty") can also be acceptably written as schoen, and thus avoid confusion with schon (meaning "already").  Likewise, Fräulein ("Miss") may be written as Fraeulein, and über ("over") as ueber, without changing either meaning or pronunciation.

ß — While this might appear to be a misplaced Greek b (beta), ß is actually an alternate form of ss (also sometimes written sz, and therefore called ess-tzet).  When encountered in a word such as muß or außer, the ß is pronounced as an unvoiced ss, as in massive.  Likewise, on keyboards without the ß character, it should be transliterated as ss.  It is a mistake to write a German word, such as groß (meaning "large" or "grand") as grob (meaning "coarse" or "rude;" it should be spelled gross.  That way we avoid such embarrassments as having our Großeltern / Grosseltern (grandparents) become Grobeltern (crude parents)!  The letter ß exists in the lower-case form only; when writing a word in all-capital letters, one should replace ß with SS.  Thus, Straße may (perhaps on a subway station sign) become STRASSE (never STRAßE).

Another facet of written German that often strikes non-Germans as peculiar is capitalization.  All German nouns are capitalized, while modifiers typically are not.  Thus, "The American man rides in a German car" is properly written in German as "Der amerikanische Mann fährt in einem deutschen Auto."