After leaving Yam Suf (Red Sea) and walking for three days in the desert, Am Yisrael (the People of Israel) are out of drinking water and all they can find is bitter water.
In a great miracle, Hashem (G-d) turns the bitter water to sweet drinking water. Following that, the Torah tells us that at that place Am Yisrael were commanded several laws, and also received a promise that any disease that Hashem put upon the Egyptians, He will never put upon Yisrael.
Why is this the first experience Hashem takes them through in the desert? What does the fact they were out of water have to do with the punishments brought upon Mitzrayim (the Egyptians)? Why is it here where they received their first new mitzvot (commandments)? And how is all this connected to Moshe's (Moses) sister Miriam?