The life loss is based on the power of the sacrificed creature as it last existed on the battlefield.

You can sacrifice any land with the land type Swamp or Forest, not just ones named Swamp or Forest. For example, you could sacrifice two Overgrown Tombs.

Jarad's first ability applies only when Jarad is on the battlefield.

To activate Jarad's last ability, you must sacrifice two lands. It doesn't matter what other land types the two lands have, as long as one is a Swamp and one is a Forest. Even if one of the lands you sacrifice is both a Swamp and a Forest, you have to sacrifice another land that is a Swamp or a Forest.