There are 3 options I would do here, not sure what supplies you have on hand though.
1) Use edible markers (this would be my preferred, but I don't have these yet either)
2) Thin down your gel food coloring and paint it (this is what I end up doing most of the time)
3) with icing use a tip 1 or 1S, just make sure your icing is VERY VERY smooth and no little pieces of anything in it or it will clogg up.
HTH
As for writting with icing the best tip I can give you is to make sure you are using your whole arm. I have found when working with other decoraters that we all move our arm from our shoulder down instead of from our wrist like when you write with a pencil/pen.
My writing on cakes and plaques leaves alot to be desired. I have been using edible images to put writing on my plaques. If you don't have that option available, you could try the pin prick method. Print out a font you like from your computer. Lay the sheet of paper on top of a piece of foam and use a pin to punch through the letters. That will leave little bumps on the backside of the paper. Then you can take that paper, gently lay it on the plaque and press it. It will leave a pattern on the fondant that you can trace. This will only work if your fondant has not dried. I took a look at your picture and saw the plaque so the pin prick method probably isn't going to work. Another method you could try would be a gel transfer. You might be surprised at how good your writing will look on a plaque. I have also printed a font from the computer about the size that I want my writing to be and layed that just underneath where I am writing. That works as a guide for me to help keep my printing straight and the size I need it to be. Good luck. Let us know how it turns out.
Hi,
I'm totally with you on this one. I still need much practice with writing on cakes so I asked Cambo on this site as her work is very good. She gave me a nice and easy suggestion. On Powerpoint make a greeting then cut it out carefully and using a glue stick mount it on a piece of foam. You can then make a 3D effect by using another color behind it (check my photos for the Farewell cake). An alternative method again is the edible markers or I also found a Tip #2 to work fairly well. Adding a little Piping Gel also helps it flow smoothly. Good luck - I'm sure you'll do fine. 
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