Circuit diagrams show how electronic components are connected together. Each component is represented by a symbol and a few are shown here:

Circuit diagrams and component layouts
Circuit diagrams show the connections as clearly as possible with all wires drawn neatly as straight lines. The actual layout of the components is usually quite different from the circuit diagram and this can be confusing for the beginner. The secret is to concentrate on the connections, not the actual positions of components.

I installed a software that let me use the PC as an oscilloscope , but
it does not mention how to connect the circuits to be tested with the computer an through which port.Does anyone lead me?
I mean how can I connect that cable to
the circuit ?

This is an important action when checking the circuit performance for analysis or when you repair a circuit/electronic device.

When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer chip. This can seem confusing – where should you connect the second multimeter lead?

Measuring voltage at a point

• Connect the black (negative -) lead to 0V, normally the negative terminal of the battery or power supply.
• Connect the red (positive +) lead to the point you where you need to measure the voltage.
• The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points.
• You may wish to fit a crocodile clip to the black lead of your multimeter to hold it in place while doing testing like this.

Block diagrams are used to understand (and design) complete circuits by breaking them down into smaller sections or blocks. Each block performs a particular function and the block diagram shows how they are connected together. No attempt is made to show the components used within a block, only the inputs and outputs are shown. This way of looking at circuits is called the systems approach.

Power supply (or battery) connections are usually not shown on block diagrams.

source: http://www.kpsec.freeuk.com/study.htm

This is the block diagram of the most of regulated power supply.

Notes:

• Transformer – steps down 230V AC mains to low voltage AC.
• Rectifier – converts AC to DC, but the DC output is varying.
• Smoothing – smooth the DC from varying greatly to a small ripple.
• Regulator – eliminates ripple by setting DC output to a fixed voltage.