Tujuh Maret has got to be one of the best Indonesian restaurants I've ever been to in Amsterdam. Word on the street is that the best in Holland are actually in The Hague due to a high population of Indonesians. If you don't make it there, and you should try to, because it's a great city not far from the sea, you should hit Tujuh Maret in Amsterdam.
Please stay away from places in the Leidseplein area or Rembrandtsplein. You should walk up the Utrechtsestraat (which incidentally runs into the Rembrantsplein) for some great Indonesian. Utrechtsestraat takes you into a nice residential area with bodegas, flower markets (on every canal), and tons of little boutiques. Nestled in between is Tujuh Maret. The atmosphere is decent but definitely not luxurious, since it mostly is a takeout place for locals. Still, the restaurant is cozy and the wait staff is super-friendly.
If you're new to this cuisine, ordering rijstaffel (or "rice table") is a good way to taste many dishes at once. It consists of tiny portions of 10 to 20 dishes, from mild to hot. If you'd rather try one dish and you're new to Indonesian, a good choice is sate with spicy peanut sauce. It's a very safe dish, but incredibly tasty. You can get this in chicken or in pork. For those more adventurous, try the chicken roedjak, a spicy red pepper-and-coconut sauce, or beef rendang, a spicy sauce with stewed beef. Make sure to eat a lot of Krupuk (delicious shrimp chips) and a nice beer to complement the exotic flavors. Indonesian cooking is actually not too complicated, and if you choose a base meat that you're comfortable with, the sauces will not scare you off. I've been much more afraid of Chinese and Malaysian cooking!
I highly recommend trying this while in Holland, since you can't just survive off of Dutch pancakes and cheese sandwiches the entire time!