- Microsoft Visio
You can start, by choosing
Template: UML Model Diagram (Metric) or UML Model Diagram (US units)
You can look at [here] for starting Poseidon.
This link is designed for a Lab of CPSC211 (at UBC)
There are also some captures.
Frankly, It was not handy for me at the first trial.
Anyway, this is for free.
I used to draw with Microsoft Visio because it was installed on computers at UBC.
You may not use Paint because it is not made especially for drawing UML Diagrams. But if you can not use anything above, you may use Paint, Microsoft Office Word or anything you feel suitable.