Being a nation of islands, you can really only get to Indonesia by air or by sea.
Indonesia By Air
Although there are actually several international airports in Indonesia, most tourists coming by air enter through either Jakarta or Bali. Many international carriers service one or both destinations. Additional cities can be reached by air from Singapore, where flights are often cheaper. You may be well served to get the cheapest flight you can to Singapore, then book onward travel from there.
If connecting to or from domestic flights and international routes, be advised that almost all of Indonesia's domestic carriers are notoriously late. Allow yourself a long layover where necessary.
Indonesia by Sea
One route to Indonesia, which is popular with Asians, is through Singapore to the small islands of Batam and Bintan just off Sumatra. Batam has an airport if you want to move quickly on to Jakarta by air.
You can also get a ferry from Penang in Malaysia to Medan at the north end of Sumatra.