Here's the general rules of thumb I follow when deciphering a schematic/PCB:

1) Ask yourself: what's the application? What should this board be capable of doing? If you can't answer this question from the start, it's much more difficult to frame further analysis.

2) Ask yourself: what are the inputs? What are the outputs? You don't need to analyze them in detail immediately; focus on the enumeration. USB or UART? I2C or SPI? Switches or plugs? Get a model in your head (or on paper) of what protocols will be sent to and from the board.

3) Ask yourself: what are the most important elements of the board? You're usually looking for schematic blocks with lots of pins. Identify what those blocks are and what their purpose is.

4) Ask yourself: where does the energy come from? How are the board's components getting powered? If the power system doesn't work, nothing will, so it pays to understand this sooner rather than later. Enumerate the power rails. Are all rails coming from off-board, or are there power supplies on-board? If the latter, where does they get their energy from, and where does it go to?

5) Ask yourself: where do the input and output pins go to? Think of how information propagates through the board.

If you've followed these steps, you should be able to describe the functional operation of the board and understand why large components are generally connected the way they are.

Inevitably, you'll encounter smaller, "discrete" elements like resistors, capacitors, inductors, diodes, transisors, ferrite beads, and more oddities which may leave you perplexed. Sometimes, these will be included as required supporting circuitry for an IC, so it may pay to dig through the datasheet and look for recommended layouts and required circuitry. Sometimes, these elements are used to improve a design in a particular way (pullup/pulldown resistors, voltage divider resistors, decoupling capacitors, debouncing capacitors, DC-filtering capacitors, and many more). I'll provide some basic ways to identify the ones I've listed above.

- Pullup/pulldown resistors: often connected to a trace carrying data. Ensures the trace isn't left in a "floating" state. One end on trace, the other on VCC (pullup) (if DC signal, will be same voltage as line voltage) or GND (pulldown). Typically ranges from 1k-10k ohms but may leave this range, depending on your requirements.

- Voltage divider resistors: a cheap way to get a lower voltage. Imagine two resistors connected in series. Something is hooked up to the connection between the two resistors. One resistor is often hooked up to VCC, and the other is often hooked up to GND. The voltage divider formula will tell you what the voltage is between the two resistors.

- Decoupling capacitors: typically located near where things are happening (input, output, IC). The idea is that they act as local energy buffers. For an IC, this means it doesn't always need to reach all the way to the battery or supply for juice; it's got some nearby. This is the "decoupling." Expect 0.1uF - 10uF values. For certain applications, different types of caps (tantalum, electrolytic, film, ceramic, etc.) may be used to improve frequency response, so keep an eye out for groups of caps of different values.

- Debouncing capacitors: usually used for cheap input devices like certain pushbuttons. One end of the cap is on the input line, and the other is connected to GND. Springiness in the input device can cause the contact to imperfectly connect and fluctuate the line voltage, creating an undesirable AC component to the signal. These caps reduce sensitivity, so you're probably going to see caps around the 10pF range.

- DC-filtering capacitors: the compliment to the debouncing cap. Often seen on antenna lines. In these situations, you care most about the signal's AC component rather than the DC. The cap will be found bridging the signal line. The specific value will vary based on your application.

Ultimately, there are many little things you'll need to learn, especially on complex boards. However, I hope this information gives you a mindset by which you can begin to approach even the most complex boards.