You'll get the best idea from the Text & Font tab, which contains some of the most common properties. So, select that, and select the "H1" from the list or tree. Click the [...] next to the background colour and click on some of the coloured tiles. With any luck, the heading of the sample page should change, and you're using CSS Preview.
Where a property has two controls, the first one is for the value, and the second one allows the selection of units. The second control also allows selections of some keyword values, e.g. inherit
A few properties have more complex controls. The colour dialogue offers several ways to select colours, most of which are pretty straightforward. If the colour is not applied to the document straight away, select the "Use This Colour" function. This is provided to help on slow machines where rapid changes in colour may cause laggy updates.
If a property does nothing, this could mean that either your browser doesn't support this property or value, the value you selected is the default, or some other property is needed to make this one take effect. For example, IE doesn't do position: fixed, and setting a border colour will not have an effect without a size and style.
