I used a 18 volts solar panel , though many people here will recomend you to use 9 volts solar panel
Advantage of 18volts panel is that you will get the voltage and required charging current even in the dim light ,(if you use smaller voltage panels , though you get voltage , the current may be too less)
Step 1: CIRCUIT DIAG
1. 7805 ic------------1 cost about 0.4 usd
2. 10mf cap --------2
3. soldering iron and plain solder board ,cost about 3 usd
If you like to add a LED , then one led and 3k ohm resistor 1/4w.
Please take care of the polarity , ie , positive of capacitor connected to + of supply .same for LED too.
Note , i made a small loop so that i can hook the wire to it , so that i need not have to solder and de solder again.
Hook I made from LED wire , which is very strong , and easy to solder .
you can keep this in a small box or on a wooden board to be safe .
Solar panel is best variable voltage source to give to input and check the constant output voltage of the circuit.
Please note : you solar panel wattage given by manufacturer is in ideal condition , you can never achieve that ,so the current drawn from the panel will be much less in most of the time , so better to go for higher rating , so that you can get power even at low light..