How can I use the 2N3904 transistor to amplify the current? Can anyone give me an example?
You need to explain exactly what you are trying to do. Your question indicates that you don't have a clear understanding of the behavior of voltage and current (Ohm's law for starters).
Current is dependent on the load impedance. You don't just put some current into a device like a transistor, and then get some greater current out. The transistor must be driving some load - what is that load? What are you trying to do? Is the load purely resistive, or is it a relay coil, filter, resonant circuit, small motor, LED, speaker, etc.