Steps refer to a specific interval between 2 notes. To explain the concept of steps we'll use a keyboard as shown below.
2 closest adjacent notes on a keyboard are called half steps, and 2 examples are shown in red. For instance the C on the far left and the black key beside it are half a step apart.
A whole step on the other hand refers to notes which are 2 half steps apart, i.e. 2 keys apart on the keyboard, and examples of these are shown in blue.
A more commonly used terminology for intervals is in terms of tones and semitones. A semitone has the same interval as a half step, whereas a tone has the same interval as a whole step.
Accidentals are used to change the original pitch of a particular note. We will discuss 4 different types of accidentals in this section, which include the double flat, flat, sharp and double sharp. The natural is another type of accidental, which will be discussed later – it's just shown here for completeness. Make sure that you are familiar with steps before you read on as this will be essential for understanding the concept of accidentals.
The flat is used to shift the pitch of the note down by half a step. For example look at the far left example in the above image. The quarter note shown on the staff is a G. However as it has a flat shown next to the note this note would in fact be half a step down, i.e. the black key to the left of G. This note is called a G Flat, which can also be denoted as Gb. The natural note is shown in red, and the flat version of the note is shown in blue.
Note that a flat note does not always have to be on a black key. For example the last example shows an Fb (F Flat). As a flat note is half a step down (i.e. the nearest key to the left) this would be an E, which is in fact a white key. The Fb is not usually used however, as it can just be denoted as an E.
A sharp is the opposite of a flat, in the fact that it shifts the note half a step above usual. Therefore when playing a sharp note, you would play the nearest key to the right of the original note. This is clarified in the examples shown here.
Remember that the keys highlighted in red represent the natural note, whereas the blue shows the sharp note. When writing text, a sharp note can be denoted using a "#" next to the note. For example a G sharp can be written as G#.
Double flats and double sharps are basically the same as flats and sharps. The only way they differ is how they shift the original pitch of the note by a whole step rather than a half step. They're represented on music using the symbols shown above. However it can also be represented using a "bb" (double flat) and an "x" (double sharp).
The examples shown should help clarify the concept of double flats and double sharps. Notice how the notes represented on the music represent pitches that are a whole step below/up (i.e. 2 keys apart) from G.
You may have noticed that some pitches have more than one name. For example take a look at the above image. The key highlighted in blue can be referred to as G# or Ab, and both would be a legitimate name for the note. The way that notes can have more than one name is called enharmonic spelling and is a very common occurrence.