The key of the music refers to a set of 8 notes that always go together (you can hear them when you play a scale). The key of C goes from C through D, E, F, G, A, B and C again. Other keys have sharps and/or flats (these are the black notes on a piano).
Sharps and Flats:
A sharp note is a semitone higher than a note, and is written as _# , e.g. C#;
a flat note is a semitone lower and is written as _b, e.g. Bb.
There is no semitone between B and C or between E and F, so the notes are, starting from A: A, Bb (or A#), B, C, C# (or Db), D, Eb (=D#), E, F, F# (= Gb), G, Ab (or G#), then back to A...
Note that Bb is the same as A#, and so on. If you look at the Key Transposer , you will see this sequence. There is a key for each of these (A through to G#/Ab).
Each key has a set of chords which are built on notes in the key.
The main or "tonic" chord in the key has the name of the key, e.g C is the main chord in the key of C. (It is built on the tonic note plus the 3rd and 5th note of the scale).
The "dominant" chord is built on the 5th note of the scale- so in the key of C, this would be G. The dominant 7th chord (e.g. G7)is often used, as this seems to lead your ear back to the main chord (the dominant 7th chord adds in the 7th note, but flattened).
The "sub-dominant" chord, e.g. F, is the other most frequently used chord. It is based on the 4th note of the scale (with the 1st, 3rd and 5th notes added).
Then there are minor chords, e.g. Am (also written as Amin or sometimes just a).
There are also various other less-commonly used chords, e.g. major 7th, (maj7), diminished (dim), and so on... but you'll pick those up later on.
You can play and sing in different keys, depending on what pitch suits your voice- i.e. if you have a high or low voice, male or female. You need to try out different keys to see which suit you.
Some songs are easier to sing than others, because they have a narrower range of notes in the tune, or the tune is more regular and predictable. If you're not used to singing, try to find some easier songs to practise on (the songs on this site in the easy range are good for this, too).