The large life forms on earth today are only possible because of photosynthesis, a method plants use to harness the sun's energy. Most life on earth either photosynthesizes or gets its energy from organisms that do. It's very unlikely that photosynthesis would power the first cells, it's such a complicated process. But it's easy to imagine that chemical energy would be available. Chemical energy is released when water reacts chemically with rocks.
Scientists believe earth's first cells got their energy from such reactions.
Likewise, they expect that life on other planets would use geochemical energy too. While energy from the sun is abundant, geochemical energy is scarcer. That means if life exists on these other planets, it's probably less plentiful than here on earth and finding it will take careful planning. Most people who go out in their back yard and pick up a rock don't come up with a fossil. The key is to target areas that might be likely to have evidence of life if it was there, and target it so well that in fact a little bit of luck is involved.