Wiring a layout is a very important thing to get right; your entire railroad depends on it! There are several wiring schemes to choose from. First, you can wire for single train control, where you can run one train at a time on one set of tracks. Next, you can add in blocks, or electrically isolated sections of track, where you can park locomotives while other trains run. Or, you can create a minimum of 4 blocks on your main line, and when running them from two powerpacks, create cab control. With cab control you can operate two locomotives independently on one set of tracks. Lastly, you can wire for DCC, or digital command control. Each locomotive gets a decoder inside of it, and just one set of wires is run to the tracks. Trains are assigned identities based on their recievers, and users can use handheld throttles and control any locomotive independent of all the others. Advanced modelers link their DCC systems to their computers for even more fun!
Powerpacks transform your household AC current into DC that your trains like. There is a variable resistor that works as a throttle so you can control speed, and most powerpacks have a direction switch. Make sure to connect your tracks to the DC side of the powerpack, and accessories go on AC. Newer powerpacks are solid state, meaning they use transistors instead of rectifiers to convert the current. Solid state gives more precise control over the speed.
When shopping for a powerpack, or when replacing the "starter" junk unit that came with your trainset, expect to pay about $30 or more for a decent product. MRC makes a variety of models, and they're improving every day. Some models feature sound and lighting units as extras. Make sure to shop for how many amps the powerpack puts out. Estimate that each locomotive draws about .5 amp, but more is always better. This is the main reason why these powerpacks excel over "starter" models- the amperage is much smaller on the starters. You might encounter "momentum" switches. These allow you to simulate the weight of a real train; it takes a long time to accelerate and even longer to stop. Many momentum models also feature "brakes," which simulate the brakes on a train.