1,000 せん (sen) 千
2,000 にせん (nisen) 二千
3,000 さんぜん (sanzen) 三千
4,000 よんせん (yonsen) 四千
5,000 ごせん (gosen) 五千
6,000 ろせん (rokusen) 六千
7,000 ななせん (nanasen) 七千
8,000 はっせん (hassen) 八千
9,000 きゅうせん (kyuusen) 九千
10,000 いちまん (ichiman) 一万
Million ひゃくまん (hyakuman) 百万
10 Million せんまん (senman) 千万
100 Million おく (oku) 億
1 Billion じゅうおく (jyuuoku) 十億
Note that 3,000 and 8,000 are pronounced "sanZEN" and "haSSEN," respectively. As for higher numbers such as 10,000, you can see that the kanji is using 一 for 1 and 万 for 10,000. 20,000 would be 二万. One million is literally 100 10000's (100 x 10,000 = 1,000,000), hence the kanji for that is 百万. 億 (oku) means 100 million, so 10 of those (十億) would equal 1 billion.
Million ひゃくまん (hyakuman) 百万
10 Million せんまん (senman) 千万
100 Million おく (oku) 億
1 Billion じゅうおく (jyuuoku) 十億
Note that 3,000 and 8,000 are pronounced "sanZEN" and "haSSEN," respectively. As for higher numbers such as 10,000, you can see that the kanji is using 一 for 1 and 万 for 10,000. 20,000 would be 二万. One million is literally 100 10000's (100 x 10,000 = 1,000,000), hence the kanji for that is 百万. 億 (oku) means 100 million, so 10 of those (十億) would equal 1 billion.