Hey there,
Assuming that you are flying to Incheon airport (if you are flying in from overseas then you most probably are) you would be best off taking a bus. The airport busses are extremely convinient and they take you anywhere in Seoul. There are quite a few busses that you can take, so I'd need to know what do you want to do in order to give you proper directions.
If you want to just stroll around the city center I'd recommend taking the bus 601 to City Hall or Jongno. From there it's an easy and short walk to Insadong, which is the touristy area of Seoul. And when we're talking about touristy, well, Cambodia is more touristy then Seoul.
Depending on the time you arrive you should allow yourself at least an hour and a half to get to the city. The busses leave the airport every 15 minutes. I found the bus trip actually quite scenic (I know it's hard to believe but in terms of rides to the airport Seoul-Incheon is actually really nice!)
You can buy the bus ticket on your right when you come out of the terminal. It's all very well signposted.
If you have any questions please don't hesitate to pm me.
Have a good time in Cambodia!
merja