The first space on the bottom of a treble clef staff is an F. Because the alto clef is one note name higher, this note becomes a G in the alto clef. Even though the written pitch is one note higher, the actual sounding pitch is one octave lower. The first space of the alto clef is the distance between the first line and the second line in the clef. In relation to middle C, this G is the first G below middle C. Middle C is the C that is in the middle of the bass and treble clef piano staves.
The second space from the bottom of the alto clef is a B. It is not necessary to think in terms of the treble clef once you know what the bottom staff is. The lines on the alto clef move in thirds. This means that each space will be exactly a third higher than the previous space. This B sits right next to middle C.
The third space from the bottom is the first pitch to move beyond middle C on the staff system. The third space named D in the alto clef is the first alphabetical note that comes after C. For an instrument that plays in the alto clef, this is a comfortable pitch to play in the middle of the range.
The top space, or fourth space from the bottom, is the pitch F. F is a fourth higher than middle C and appears in the treble clef as an F on the first space. This space is the last space on the staff before notes appear above the staff. Often, when notes in the alto clef go too high out of the staff system, they will continue into the treble clef.