Hello Jorge
Illustrator is best if you plan on printing. so if that's what you want I think I can help. I don't use the cloud (I don't believe in buying time it's got to be the real thing for me) so I can only advise you on the CS versions that I have for this one I'm using CS5 but its the same instructions for both CS4 and CS6. You didn't say what this was for so I assume its some kind of sign. I'm also going to assume you know how to use Illustrator so I won't be explaining every single step.
Open Illustrator go to File/new - put your dimensions in the box - in the units box - inches. CMYK in color mode if this is to be printed or RGB if its strictly for the web. I like my raster effects high I use 300ppi and default view. Name and save your document.
Go to Effects on the drop down menu and select document raster effects. Under background make sure that Transparent is selected. This is your background so name that first layer background. Then create a new layer above it. That will be for the box you'll be drawing.
Make sure new layer is selected and create your text box. There are several ways to do it either with the rectangle tool, the pencil tool or the pen tool. I would use the pen tool because it makes it real easy to make those concave curves on the corners. Also set your stroke thickness in the stroke panel - select the box with the select tool. Click the stroke tab in the panel box and choose 1 or 2 for the stroke thickness.
While its still selected choose the color you want for the box in the color panel. You can always change that too.
Create a new layer above the box layer and name it text. Select the text tool and the font you want to use and type your text. Position it within the box however you want by using the select tool.
that's pretty much it unless you want to add other effects or artwork. Save your file.
Hope this helps.