I'm new to electronics. I have an Arduino and love it. I'm in the beginning stages of understanding and building an H-BRIDGE. My question is:
If i'm using a transistor as a "switch" how can I tell the amount of voltage I need to pass to the base in order to completely turn it "on".
For example http://www.sparkfun.com/datasheets/Components/BC546.pdf
Vbe sat or Vbe on, middle of page 1 datasheet (not really sure which but they are both in the same area) or 0.7v is typical
just wondering, how much does sparkfun want for that 10 cent transistor?
I'm new to electronics also but I'm going to take a shot at answering this question. I want to show off what I've learned in the last couple of weeks 
The 'Base-Emitter On Voltage' parameter in the datasheet denotes the voltage needed to turn on the transistor. Apply the voltage at the high end of the range (700 mV) across the Base/Emitter to use the transistor as a switch. Doing this makes current flow from the Colletor to the Emitter.
The amount of current that flows from the Collector to the Emitter is controlled by how much current is fed into the Base. The 'DC Current Gain' (Hfe) parameter in the datasheet denotes the ratio of base current to Collector/Emitter current. At high currents the gain will be on the low end of the range, so lets use the 110 value.
To get the maximum amount of allowable current to flow through the collector (100mA) you should construct your circuit so that there is 100mA/110 = .9 ~= 1mA of Base/Emitter current.
Hope that helps.
And apologies in advance if I have given you bad advice
Note that bipolar transistors (as opposed to MOSFETs) are turned on by CURRENTS flowing from base to emitter rather that voltages. You need at least 0.7V to overcome junction voltages (sort of like an LED), but you need to limit the current with a resistor (sort of like an LED), and the amount of current you allow to flow through the junction controls how much current flow through the C-E path. To act like a switch, the current in the base (IB) should be significantly greater than (IC / hFE)
For your BC546, max IC is 100mA, and min hFE is about 100, so a base current of 10mA will make for good switching. (10*100 >> 100)
just wondering, how much does sparkfun want for that 10 cent transistor?
It would be $54.76 !!! (FedEx overnight)
Ridicules !
I think what i'm looking for is a MOSFET. Seems easier to use with an Arduino?
1 Volt should be fine.
Yes, a MOSFET is turned on by a voltage (unlike a bipolar transistor.) They would theoretically be "easier" to interface to an Arduino except that many MOSFETs require about 10V to turn on all the way. While the Arduino's output current capability of 30+ mA is sufficient to turn on most bipolar transistors, the output voltages of about 5V is not enough to turn on (completely) many MOSFETs. Also, MOSFETs tend to be more expensive and more delicate (subject to ESD damage) than equivalently powerful bipolar transistors...