What is voltage gain of a transistor amplifier and how is it affected by frequency?
(Added much later) Zeroth, a bipolar transistor is actually a very complex device, when it comes to the actual device physics.  Voltage across the emitter-base junction (base bias) determines the current into the emitter, which all goes into the base region.  The device construction determines how much of that current recombines in the base (and becomes base current) and how much continues on into the collector.  (The current "splits".)  The ratio of collector current to emitter current is called \$\alpha\$ (alpha).  \$\alpha\$ generally ranges somewhere between 0.95 and 0.995.

Usually, ratio of base current to collector current is more useful, as that's the DC current gain (\$\beta\$) (beta) of the transistor.

A bit of algebra will show that

\$\beta = \frac{\alpha}{(1 - \alpha)}\$

or, alternatively, 

\$\alpha= \frac{\beta}{(\beta + 1)}\$

For example, \$\alpha = 0.95\$ gives \$\beta = 19\$, and \$\alpha = 0.99\$ gives \$\beta = 99\$.

First, a transistor is usually modeled as a current-controlled current source.  Voltage gain is a function of \$\beta\$ and the circuit values.  Current gain does vary with operating parameters.

The venerable 2N2222A specifies a minimum \$\beta\$ of anywhere between 35 and 100.  At Vce = 10V and Ic = 150 mA, the device advertises a minimum \$\beta\$ of 100, and a maximum of 300.

Second, to a first approximation, the transistor may be regarded as a fixed-gain device at frequencies from DC up to what might be called the "corner frequency".  The corner frequency is the gain-bandwidth product \$f_T\$ divided by transistor's \$\beta\$.

The venerable 2N2222A advertises a minimum \$f_T\$ of 300 MHz, measured at Vce = 20V, Ic = 20 mA, and f = 100 MHz.  With a minimum DC current gain of 100, the corner frequency will be at least (300 MHz)/100 = 3 MHz.  Below 3 MHz (or above, depending on the device and the operating conditions), the device has a fixed current gain.  For frequencies above 3 MHz, the gain rolls off.  At 30 MHz, you would expect a minimum current gain of 300 MHz / 30 MHz = 10.

Your mileage WILL vary.  You design conservatively, and you test your circuit, ideally with several transistors.

This gets you started.