Hydrogen is an element that likes to be in first place. It is the simplest element, the lightest element, the most abundant element in the universe, the first to come into existence, and you will also see it sitting proudly at position number one on the periodic table of the elements. In its most basic form it contains a nucleus, which consists of just a single proton, with a single orbiting electron, and it is this form that makes up around 99.985% of the naturally occurring hydrogen.
Heavier, and far rarer isotopes of hydrogen also exist, and these contain one or more neutron along with the single proton in the nucleus. Hydrogen-2 (deuterium), with a single neutron, makes up about 0.015% of the naturally occurring hydrogen, for example. Helium-3 (tritium) is a very rare, and also naturally occurring, but unstable, radioactive isotope with a half-life of around 12.3 years.
Hydrogen makes up around 75% of the ordinary matter in the universe and over 90% of the atoms. It can exist as a diatomic gas, in a simple atomic state, or as a plasma. Clouds of molecular hydrogen are thought to be the birthplace of stars. Meanwhile the interior of stars are giant fusion reactors in which hydrogen atoms are fused into helium, giving up energy in the process. Proton-proton reactions in stars are also a result of hydrogen. Hydrogen as an atom is also found in the interstellar medium.
Hydrogen gas is rare on Earth as it is so light that it can escape our gravity. However, hydrogen in the atomic state does exist in abundance on the surface of earth in a variety of compounds. The most notable of these compounds being water and hydrocarbons.
Hydrogen also exists in that fascinating form of matter known as a plasma, in which the proton and electron are not bound to each other, causing these atoms to be highly conductive and emissive, and highly susceptible to magnetic and electric fields. The beautiful phenomenon known as the aurora, for example, is the result of the charged particles of the plasma in the solar wind interacting with the magnetosphere of the Earth's atmosphere.