Im 5'8 and 138 pounds... Im think im too skinny. What do you think?

I think your perfect, a couple inches taller then me and a couple pounds more thats just how it should be =D
If you want to gain more weight/look more athletic take up some sport or exercises and ton up a bit, it'll put some more weight on you simply because muscle weighs more and if your arms/legs look thin - myn were getting very - you would be able to tone them into looking good - but too much can look strange too XP

I think you're just perfect.

According to my research, an average 5'8 female should weigh between 126 and 154 pounds. You are right in that range so continue to do what you are doing as far as diet and exercise go.