Learn Icelandic: Hér er einn tómatur.

In Icelandic, the numbers 1, 2, 3, and 4 change their form to match the gender of the noun they are counting. Because tómatur is a masculine noun, we must use the masculine form of "one", which is einn.

How can I tell that tómatur is a masculine word so I know to use einn?

A great hint is the ending of the word! Nouns that end in -ur in their basic dictionary form are very often masculine in Icelandic. Since tómatur ends in -ur, it takes the masculine number einn.

Does einn mean "a" as well as "one"?

Icelandic actually doesn't have a word for "a" or "an" (an indefinite article). If you just want to say "Here is a tomato", you drop the number and simply say Hér er tómatur. You only add einn when you want to specifically emphasize that there is exactly ONE tomato.

Is the word order exactly the same as in English here?

Yes, in this specific sentence, the word order is identical to English. Hér means "Here", er is "is", einn is "one", and tómatur is "tomato".