There are lots of ways to roll d100. As noted by JayBonci, you usually roll two d10 and use them as the two digits. Before rolling, you should say which die is the first and the second digit, so you need two different dice.
It is quite easy to find d10 dice with 00, 10, 20, ... on their sides, so you can use them together with normal d10 to roll d100.
Our club uses a simple convention to roll d100. We roll a d10 and a d20 together, getting the first digit from the d10 and the second from the last digit of the d20. There's no need to call the dice before rolling (as we always use this technique). It is also useful because quick and dirty dice packs often have only a die of each kind.