You need to buy one or more transformers with sufficient conbined power for the total wattage of the lighting that you have selected. The total wattage of the lighting connected to each transformer must not exceed its rating. For example a 60W transformer can run 3 x 20W lamps, or 30 x 2W lamps, or 3 x 10W lamps plus 15 x 2W lamps.
The total wattage of the lights that you have in your basket so far is: 0W
Now go to Step 3 and choose the cables that you need