Look at File:Edit:Net Classes. There you can specify a default width, say 10 mil wide with 10 mil clearance and 12 mil for vias.
You can define wider ones, say 20 mil for power, 10 mil clearance, and 24 mil for vias.
Right click on the traces and under Property select the trace type you want.
"i dont have any experience about eagle and i am trying to design a pcb. It will be a commercial product so it needs to be perfect."
Work with an engineer the first time.
Download the DRU file and CAM file from someplace like iteadstudio.
Run the Error checking on the schematic, fix those errors.
On the board, select View:Grid and set the spacing to 2 mil.
On the board, use the Polygon and draw polygons around the board dimension on the top & bottom layers. Right click and Name then GND.
Run the Rats Nest and see what is left to connect. You may have to take narrower traces off PCB pins to connect to wider power traces.
Run the Error checking on the board. Fix those errors.
Work with an engineer the first time.