1: You need know wattage = current * voltage at first. for example: 12V 3A power supply = 12*3=36 watt.
2: You need know the LED type of the LED strip you have
SMD5050 0.24W/LED ( SMD5050 = 3 SMD3528)
3: You need know the density and length of the strip you will buy.
for example, if you buy 60LED/meter SMD3528 in 3 meters,http://www.ledlightsworld.com/smd-3528-flexible-led-strips-300-leds-p-90.html so total 3*60=180LEDs each LED consume 0.08W so this strip will need 180*0.08= 14.4W
then you check the power supply, you can find there have 12V 1A 12W and 12V 2A 24W and 3A 5A version, in this case, choose the wattage bigger than your strip’s consume wattage. so choose the 12V 2A 24W power supply for it. you can buy the power supplies at http://www.ledlightsworld.com/led-power-supply-110220v-ac-to-12v24v5v-dc-p-1000706.html
any problems, please feel free to contact us at firstname.lastname@example.org we will reply you within 1 working day