Beware the problems of colour matching. If your printer came with a colour calibration tool, which tries to make sure the printer produces the same colours on the page as appear on the screen, use it. It will save a lot of effort trying to get just the colours you want on a card. Colour calibration isn't an exact science and depends on the ink and paper being used.
If there's no calibration routine, or if the colours you expect aren't coming out of the printer, you may have to correct things the hard way, by printing a swatch of colours and using that as a guide, rather than the on-screen palettes. The easiest way to do this is to create a single page with a lot of filled rectangles on it, using a range of colours from the design program you're working with.
Use shades of the colours you expect to use in your card and identify them with their RGB values, which pop up when you hover your mouse over the corresponding colour in the Swatches palette. Print out the page and select colours for the card from the printed swatches, to be sure of getting them in the design.
Good design is never fussy and a good greeting card should stay simple. A bold illustration and a simple message is all that's needed on the front. Put your text in a text box, so you can easily adjust its position and make it a good size, say around 36pt.
Font choice is a personal thing, but for a Christmas card it should be fairly traditional - it's one of the few places it's appropriate to use a gothic font, for example. Don't use Comic Sans. The text can be coloured - Santa Claus red is good, yellow is good on a darker background, green not so good unless it's related to holly and mistletoe.
The inside of the card is a spread of two pages, where you put your message on the right-hand page, leaving the left-hand page blank. You may like to put something on the left-hand page, though, such as a poem which expresses some good feeling about the season, which you think adds to the overall greeting. Again, colour text for the main greeting brightens up the card.
The back of the card would normally take details of the manufacturer and any copyright messages. If you have any copyrights to declare or want to make up your own publisher, this is where you add details.