Welcome to the daily halacha (Jewish law). Is one allowed to go shopping for Shabbat on Friday morning before he prays Shacharit? Hacham Ovadia Yosef says that a person should avoid going shopping for Shabbat, even though it is a tremendous mitzvah (commandment). First, he should pray Shacharit as usual, and only afterward should he go shopping.
However, if a person knows there will be nothing left on the shelves, or knows that he will not have time afterward, whatever his schedule is, he should try to arrange things so that he has time. But let’s say here and there, there is a situation where a person knows that after Shacharit he is not going to have time to go to the supermarket, or that the traffic is going to be too long, and so on. In such a case, a person can go shopping for Shabbat before Shacharit, but first say Keriat Shema (recitation of the Shema). You say Shema Yisrael (Hear, O Israel), Ve'ahavta (And you shall love), Vehaya im shamoa (And if you listen), and Vayomer (And He said)—all three paragraphs of Shema, including Shema. Then, afterward, you can go shopping, and after you have done your shopping, you can pray Shacharit with the minyan (prayer quorum), the Amidah, and everything.
But I should try to be avoided where a person can have a schedule where he can go shopping normally after Shacharit. So even though a person is not allowed to do any melachah, any acts of work, before he prays Shacharit, shopping for Shabbat is such a big mitzvah, and it is something that can be done after the minyan, after Shacharit. There are a lot of people who go shopping for Shabbat before Shacharit, but just make sure you say Shema before you start driving and head out to the supermarket.