There are a couple options for the early game.
The simplest and easiest, though, is probably to just plant some trees, cut them down, and burn them for fuel. You can stick them into an IC2 generator, or (if you're into doubling ores and don't have diamonds) into a BC stirling engine. (As a side note, setting up a Sawmill, sawing the wood, then burning the planks will give you slightly more energy than charcoal.)
Once you get a little more bronze going, you could investigate peat farming - building a Peat Bog and a Turbary requires a bit of bronze, dirt and sand, but once you get that going, you'll have quite a bit more fuel than you'll know what to do with. (I recommend combining an Igneous Extruder and a Pulverizer to get the sand for that, by the way.) Peat is great, too- you can burn it in any engine/generator, not just the peat engine.
Later on, you can either pump lava for energy - although in the new release that's coming out, that is actually quite expensive - or you could set up a biomass system.
