I was just wondering because there never seems to be a beginning. Greek mythology always seems to start with just there being Uranus and Gaea with no events before that. Is there a beginning before the beginning?
Well, before Gaea and Uranus there were a few gods, but not many. In the Greek story of creation it says
In the beginning there was only Chaos. Then out of the void appeared Erebus, the unknowable place where death dwells, and Night. All else was empty, silent, endless, dark. Then, Love was born bringing along the beginning of order. From Love emerged Light, followed by Gaea, the earth.
So before Gaea, there was Chaos, Erebus, Night, and Love. Love then created Light, which in turn had Gaea. Then, Erebus and Night has Ether and Day, heavenly and earthly light respectively and Night alone had "all things that dwell in the darkness haunting mankind." This includes Death, Fate, and Sleep. Gaea then gave birth to Uranus, who she then married.
So, there were powerful beings before Gaea and Uranus. How these beings came into existence is less clear, but it seems that how it all starts with Chaos is similar to the creation story for Christians. Chaos was just there.
The site that michaelpri gives is incorrect. Here is the relevant passage in Hesiod, starting at line 116:
First Chaos came to be, but next wide-bosomed Earth (Gaia), the ever-sure foundations of all the deathless ones who hold the peaks of snowy Olympus, and dim Tartarus in the depth of the wide-pathed Earth, and Love (Eros), fairest among the deathless gods, who unnerves the limbs and overcomes the mind and wise counsels of all gods and all men within them. From Chaos came forth Erebus and black Night (Nyx); but of Night were born Aether and Day (Hemera), whom she conceived and bare from union in love with Erebus. And Earth first bare starry Heaven (Ouranos, Latinized as Uranus), equal to herself, to cover her on every side, and to be an ever-sure abiding-place for the blessed gods.
In Greek mythology there are a few different theogonies (accounts of the origin of the gods), the most "mainstream" of which is entitled simply the Theogony, written by the poet Hesiod. In this poem of Hesiod's, Gaia (the Earth), without the assistance of male seed, gave birth to Uranus (the Sky), in order for him to be a sort of dome to cover her surface, the heavens here being envisioned as something like an upside-down bowl fitted over the plate-like disc-shaped earth. Gaia then married this son of hers, just as she consorted with her other son Pontus (the Sea), who was born shortly after Uranus.
Before Gaia there was Chaos, the expanse of emptiness in the space afterwards filled by the universe. What Hesiod says about the origin of Gaia is ambiguous and can be interpreted to mean either that Chaos spontaneously became the first being to exist, after which Gaia then just as spontaneously became the second entity; or that Chaos gave birth to Gaia in much the same way that Gaia later gave birth to Uranus and Pontus.
Erebus (Darkness) and Nyx (Night) are among a few other beings who emerged from Chaos, or nothingness, in the beginning. They married each other and became the parents of Aether (the clear bright air above the clouds) and Hemera (Day[time]), who themselves married each other and likewise produced offspring.
Fragments of surviving Orphic cosmogonies (accounts of the world's origin) tell a similar story except that in them it seems to be of quite some importance that Hydros, "Water," is understood to be the basic element of existence, and thus Hydros was the original being, similar to Hesiod's Chaos. Out of Hydros silted forth Gaia, in the form of mud, who thus is the daughter of Hydros, by whom she became the mother of Chronus, "Time" (who is not to be confused with the Titan Cronus).
Confusingly the account goes on to tell us that Chronus then laid an egg containing the universe, which is to say: Heaven [Uranus] and Earth [Gaia]. Presumably in this perspective Uranus is the egg's shell and Gaia is its yolk, with other major elements of the cosmic structure being the stuff in between. Meanwhile there is an Orphic Theogony fragment which calls Uranus a "son of Nyx."
According to the Roman writer Hyginus, Chaos was produced by Caligine, "Fog" [i.e. mists of darkness]. To him, so it would seem, Chaos is male and Caligine is female. The two got married and together became the parents of Erebus and Nox (the Roman Nyx). Erebus and Nox then become the parents of Aether and Diēs (the Roman Hemera). Aether and Diēs then became the parents of Caeus (the Roman Uranus) and Terra (the Roman Gaia). Caelus and Terra also got married and their offspring correspond by and large to the Greek version of their story, the main group being the Titans, who then gave rise to the Olympians.
Part of the above is adapted from my Answer to another Question.