Halacha forbids eating before praying Shaharit in the morning, both on Shabbat and on weekdays. Eating before praying is considered a sign of arrogance, as one’s first order of business in the morning should be praying to Hashem. Of course, an ill patient who needs to eat to sustain himself is permitted to eat, as this clearly does not express any arrogance on his part. Likewise, it is permissible to drink water before praying Shaharit, as drinking water constitutes a basic life necessity. The Halachic authorities allow drinking tea, as well, which is essentially water with some flavoring, and coffee, which can be viewed as a kind of "medicine" in that helps people feel alert and refreshed. There is a debate among the Halachic authorities as to whether one may add sugar to his tea or coffee when drinking before Shaharit. Hacham Ovadia Yosef permits adding a small bit of sugar. Clearly, however, it is forbidden to drink other beverages, such as orange juice, lemonade, chocolate milk and milkshakes. Drinking these beverages before Shaharit is akin to eating before Shaharit, and is strictly forbidden.
As mentioned, this Halacha applies both on weekdays and on Shabbat. Unfortunately, many people make a mistake when they spend Shabbat in a hotel in Israel, and eat breakfast in the hotel dining room before praying Shaharit. Since they will be returning from the synagogue after breakfast is over, and lunch is not served until around 1pm, they sit down to breakfast before they leave for Shaharit. This is strictly forbidden, except in situations of ill patients or others who have a specific medical need that requires eating and drinking before Shaharit, as discussed above. If one wants to eat breakfast in the hotel, he must pray at an early Minyan and then eat breakfast after returning from the synagogue. It is likewise forbidden to take a small snack or a few drinks before praying; as discussed, one may drink only water, tea or coffee before praying Shaharit.
After one recites the Amida in the morning, there are no longer any restrictions, and he may eat and drink as he wishes.
A more complicated question arises on Shabbat, when there is an obligation to recite Kiddush before eating or drinking. Although eating and drinking are forbidden before Shaharit on Shabbat just like during the week, may one eat and drink after Shaharit but before Musaf? For example, is it permissible for one to bring a bag of candies or snacks with him to the synagogue so he can have a snack after Shaharit, such as before or during the Torah reading? Or, may one go to the synagogue kitchen during that time to take a coffee or snack?
This issue is subject to a debate among the Halachic authorities. A number of Poskim (the Shalmeh Sibur, citing from the Maharam Zakut, and Sefer Ikareh Ha’dat) maintain that the obligation of Kiddush does not take effect on Shabbat morning until after the recitation of Musaf. Therefore, once a person recites the Amida of Shaharit, he is permitted to eat and drink (as long as he does not eat a Ke’besa or more of "Mezonot" food). According to this view, it would be entirely permissible to have a snack or drink after Shaharit before Musaf.
Many other Poskim, however, dispute this position. (These include the Mishna Berura, Kaf Ha’haim, Bayit Hadash, Elya Rabba, Tosefet Shabbat, and the Hida in Birkeh Yosef.) In their view, the obligation of Kiddush begins after Shaharit, not after Musaf, and therefore, after one recites the Amida of Shaharit on Shabbat, he may not eat or drink anything until he recites Kiddush. It is forbidden even to drink water, because once the obligation of Kiddush takes effect, one may not eat or drink anything until Kiddush.
In light of this debate, Hacham Ovadia Yosef rules in Yabia Omer (Orah Haim 22:2) that one should not eat after Shaharit before Kiddush unless there is some pressing need. For example, if somebody feels a bit weak or frail and needs a snack or drink, then he may eat or drink. Other examples include a Ba’al Koreh whose voice is hoarse and he needs to drink water or suck a lozenge to be able to read properly, and somebody who needs to take medicine at a certain time with food or drink. These are all legitimate needs that allow for relying on the lenient position and eating or drinking before Kiddush. Otherwise, however, if one feels well and has no particular need to eat or drink, he should refrain from eating and drinking until he recites Kiddush. This is also the ruling of Rabbi Moshe Halevi (Israel, 1961-2001), in his Menuhat Ahaba.
Summary: It is forbidden to eat before praying Shaharit in the morning, unless one is ill or medically required to do so. Drinking water, tea or coffee is permitted, but drinking other beverages is forbidden. This applies on Shabbat, as well. After one recites Shaharit on Shabbat, he may not eat or drink anything – including water – until he recites or hears Kiddush, though in situations of need, such as if somebody feels frail or is reading the Torah and needs water for his throat, he may eat a small snack or have a drink.