A good way to create beds while you're acquiring plants is to lay down cardboard covered with compost. That will kill the grass underneath and encourage the worms to help you break up the soil.
You can mark out the beds first using a hose as a guide. Wavy lines are less formal. Corners are more formal.
Sweeps of color via repeating plants adds impact and drama. It's always a good idea to anchor any bed with hardscaping (like your pond or like a vine support) or an evergreen tree or shrub.
Think about multiseason interest. In winter, seedheads, bark color, evergreen plants. In spring, early bloomers. In summer, rely on things that will last through August. I find that autumn takes care of itself.
Find out which prolific reseeders you like - I like lunaria annua annua and tanacetum. They're easy to pull out, but they fill in nicely in spots at the end of summer when things are looking kind of ratty.
And lastly, don't forget to include some native plants in your garden. It's a great way to learn about your climate and microclimate and to have some things to rely on that don't take too much care.
That's pretty much what I know in a nutshell.
Oh, and buy yourself a little mini-greenhouse (plastic zip-covered) so you can start flats of your favorites to save money in early spring. Some things germinate so easily that it's a shame to pay somebody else for them.
