Been a while, but is there not a mention of the selection of the Istari in the latter part of
The Simarillion?
As for background on Hobbits, I'd suggest starting with the opening and closing chapters of
The Hobbit, the opening chapters of
The Fellowship of the Ring, the closing chapters of
The Return of the King, and finally the appendices regarding races and languages in
The Return of the King.
Gandalf is a chief character in
The Hobbit, The Fellowship of the Ring, the second half of
The Two Towers, and
The Return of the King. Gandalf is not human. Gandalf is analogous to an Angel. To better understand this, I'd suggest reading the two opening sections of
The Silmarillion called the Ainulindale and the Valaquenta.
All that being said, there is no quick fix to explain in one sentence who Gandalf is and who Hobbits are... but I'll try.
Gandalf is the name assumed by a lesser Ainur of Eru (the uncreated Creator) or Maia (i.e. Angel), Olorin when he is sent by the greater Ainur, the Valar, on a mission (with four other Maiar, including Saruman) in the Third Age of Middle-earth to inspire the surviving Children of Eru (the Free Peoples... Elves, Men, Dwarves, Hobbits, and Ents) to resist and defeat Sauron (a rogue Maia).
Hobbits are assumed to be a subset of humanity that appeared for the first time in Eriador (the ancestral lands of Aragorn's people) about 1,500 years before the time of Frodo. That's a bad definition, but how can I encompass Hobbits in one sentence? Tolkien's explanation at the beginning of
The Hobbit is the best... I cannot improve on that.
I'll provide a couple of links at the Encyclopedia of Arda.
Gandalf.
Hobbits.
Bon Appetit.