The blessing is recited once, before putting up the mezuzahs. You will put up the first mezuzah on one of the most important doorways in your home, such as your bedroom. As you recite the blessing, keep in mind that it applies to all the mezuzahs you will presently affix in your home.
If you can read and understand the original Hebrew, say it in Hebrew. Otherwise, you can say it in any language you understand.
Here’s the Hebrew text:
בָּרוּך אַתָּה אַדָנָ-י אֶלוֹהֵ-ינוּ מֶלֶך הָעוֹלָם אַשֶר קְדִשָנוּ בְּמִצְווֹתָיו וְצִיווָנוּ לִקְבּוֹעַ מְזוּזָה
Here’s how it sounds:
Bah-rookh ah-tah ah-doh-noi eh-loh-hay-noo meh-lekh hah-oh-lahm ah-sher ki-deh-shah-noo beh-mitz-voh-tahv veh-tzee-vah-noo lik-boh-ah meh-zoo-zah.
Here’s what it means:
Blessed are you, L-rd our G‑d, King of the Universe, Who has made us holy with His commandments and commanded us to affix a mezuzah.

What is the first blessing G-d gave the children of Israel?
Rosh Chodesh - blessing of the new moon upon exiting slavery in Egypt on route to their Homeland in Eretz Yisrael.
wasn't it the one about being fruitful and multiplying?
if this wasn't the first one then dont post my comment
Trica is asking about the first blessing while the other is brining up a commandment. Two different answers for two different questions. Just speak to your LOR