One of the big upsides of GUI is all the possible actions are right there in a nice sorted list, so you can look them over and try to figure out which one does what you want. Which part is confusing you?
For the condition, I already mentioned it was going to be a string comparison, so you can click "New Condition" from the top bar and select "String Comparison" from the dropdown list. You're working with chat strings, so the "Entered chat string" option (which is the default) should jump out to you. You can click on it and browse the other options if you want, though it should be obvious that things like filenames or ability names are not relevant to what you want to do. There's the "Matching chat string" one, but trust me that it's not what you want (the editor's descriptions are far from perfect). So select "Entered chat string".
Next is the red "Value" side of the comparison; red means it needs to be filled with something. Click it, and notice at the bottom of the box is a spot to enter text. You're checking whether a player typed "-skip", so put "-skip" in this text box.
Viola, condition done.
GUI triggering in wc3 is a wonderful introduction to programming, imo, because there's no expectation for you to constantly be looking things up. Everything is right there; no more no less. There's even a search feature for actions.
