Learn Arabic (Modern Standard): أهلا، هذا صديقي.

In Arabic, there is no word for "is" or "are" in the present tense. To say "this is my friend", you simply say "this my friend" (هذا صديقي). Sentences without verbs like this are very common and are called nominal sentences.

Why does هذا (this) sound like it has a long "a" after the first letter, even though it's not written?

هذا is one of a few special words in Arabic where a long "a" sound is pronounced but not written. It is pronounced "haa-dhaa", even though it is spelled without the first long "a" letter (alif).

How does صديقي mean "my friend"?

The base word for a male friend is صديق. To say "my", Arabic attaches a suffix to the end of the noun instead of using a separate word. The suffix for "my" is ـي (-i). So, صديق + ـي becomes صديقي (my friend).

If I am introducing a female friend, how would this sentence change?

You would need to use the feminine versions of "this" and "friend". "This" becomes هذه (hadhihi). A female friend is صديقة (sadiqa). When you add the "my" suffix ـي, the hidden "t" (taa marbuta) in the feminine ending opens up, making it صديقتي (sadiqati). So the whole sentence becomes أهلا، هذه صديقتي.