You can use your Spark in place of an Arduino. They’re mostly code compatible, and most entry-level projects shouldn’t pose a problem. There’s also the tutorials section on the forum here, which contains some neat tutorials on Spark specific functionalities. I’d suggest you just try to do the basic Arduino examples with the Spark. If you can read those examples, it should be too hard to get them to work on the Spark. You mostly have to look out for the pin declarations, but it’s really doable.
I personally think it’s better if you use the Spark with the Arduino examples, since it forces you to actually understand what’s happening. If you’re using an Arduino, it’s really tempting to just copy everything, without learning much. With the Spark on the other hand, there are some -very- tiny adjustments to be made, but they’ll ensure the lessons stick. Since the basic examples are really simple, it shouldn’t be too hard to follow along what they’re doing, so adjustments should be easy to make.
If you’re really out to get an Arduino, I’d just go for the cheapest Uno R3 I could find. Actually, that’s exactly what I did. I also started with the Spark and figured an Arduino would be easier. End result: I never touched the Arduino, and I love the !