can i add measurements to my sketch so i can use it in my planes i cant see how to add fonts
You can add dimensions using the Dimension tool.
Not sure what you mean about adding fonts. There are a few font face options but the default should probably be adequate.
Use the Dimension tool, found in the left hand fly-out menu here:
Click first on the third icon up from the bottom, which may be showing a different image (one of the ones in the arc to its right in my screenshot). Pick the one highlighted - below and to the right of the Tape Measure icon.
And use the Sketchup Learning Centre at learn.sketchup.com to get familiar with how to use SketchUp.
Most of the learning centre videos describe the desktop programme, but it is almost entirely applicable to the Free web version, except that the tools are differently laid out.
They work pretty much the same.
(Ah. As usual, @DaveR is on the ball and quicker than me!)