There is a tutorial on soldering in the "Tips & Tricks" section at the bottom of the forum contents page. If your layout will be subject to large temperature changes between summer and winter, you don't want to solder the fishplates, that will allow the rail room to expand and contract without kinking. You will probably want a small soldering iron like the one in the link
http://www.ares-server.com/Ares/Ares.asp?MerchantID=RET01229&Action=Catalog&Type=Product&ID=17107
They are available elsewhere at a much better price than this one. The one in the link is a 30 watt iron. 15-20 watt is big enough for soldering drop wires to track. You may want a bigger iron or a soldering gun for bigger soldering jobs. You want to solder a very small drop wire to the outside of the rail or to the bottom of the rail. Then drill a small hole through your roadbed, subroadbed, & benchwork between the ties adjacent to your solder joint. Drop the wire down through the hole, and solder it to a larger buss wire that leads to your power pack/throttle connection. You may melt some of the plastic ties off your track. If so, don't worry about it. You can get a bag of Campbell's wooden ties for handlaying track very cheap. I got a package and stained them with some Min Wax teak stain. They are almost a perfect match for the ties in Atlas flex track. Then you just remove the melted ties from under the track and slip in the wooden ties. Once you ballast the track, the wooden ties will disappear.
You need to install both drop wires and buss wires because the small drop wires won't carry enough current over a longer distance to run your trains, but the wire size you need for a good buss wire is as big as the rail and will look bad if you try to solder it to the rail.
One last thing on solder. You want 40/60 rosen core solder. I don't know if it is true for the rest of the country, but the last time I was in Home Depot here in California, I noticed that the only soft solder they sold was 95/5. 40/60 is a lead & tin alloy. 95/5 is 95% tin & 5% antimony. It is a much stronger solder than 40/60, we used it for soldering copper lines together in refrigeration where 40/60 would blow out from the pressure, but it melts at a higher temperature. A small soldering iron will probably not make enough heat to melt 95/5 and you will almost certainly melt ties if you try to use it. I think the reason that Home Depot switched is to eliminate the use of lead. You can get the 40/60 at any electronic supply store or industrial hardware store. Just a "heads up" when buying solder. The solder will be labeled with it alloy makeup on the package. A third solder that you might see is "Stay Brite" It is even stronger than 95/5, but it also melts at a high temp because it contains silver. I think it is quite a bit more expensive than the other soft solders, but you don't want it for model railroading applications.