Soldering doesn’t have to be difficult nor frustrating (well, at least most of the time anyway.) Presuming the tools are adequate just a simple change in technique can help a lot. That said, nothing can make up for an inadequate or filthy soldering iron.
My dad taught me to solder both electronics and copper pipes. He was an electronics technician and eventually retired as the chief engineer of a big TV station in Washington DC. There are also a couple of guys who frequent the forum who know far more than me (jump in, Craig and Mark) who are paid really good salaries for their skills and knowledge. We are all happy to share what we know.
The main thing I learned is cleanliness. That means that the surfaces to be soldered need to be cleaned of oxides. The rail will be tarnished even if it doesn’t appear that way. Try filing the base or side of the rail, use sandpaper, or whatever to make the spot shiny and clean. A little rosin flux (electronic) helps attach feeder wires. Rosin flux comes as a liquid in a bottle, a felt-tip applicator, or as a paste labeled FOR ELECTRONIC USE. It can be cleaned up with rubbing alcohol. When wiring, NEVER use a plumbing paste flux. Anything with zinc chloride or other acids will over time rot the wires (including the popular TIX fluxes and the “NOKORODE” paste flux in the US.) Save those for building brass locomotives and installing a new faucet.
Once the parts to be joined are clean and perhaps fluxed, make sure the iron tip is hot and clean. A wet sponge is commonly used to wipe oxidation off the tip. If it is clean, touch a little bit of solder to the tip to form a wet area, not a giant ball. If it doesn’t spread out evenly across the tip but forms a tight ball like a bead of rain on a freshly-waxed Porsche 911 (daydreaming here), the tip is not clean. Sometimes when this happens I will plunge the hot iron tip into the jar of flux.
The reasons for touching a bit of solder to the tip are to prove that the tip is clean, and provide a bit of a compliant surface that can help transfer heat quickly to the parts.
It can help to shape the feed wire ahead of time, and “tin” it by applying rosin-core solder to it in advance. The same is true for the rail. Otherwise, you will almost certainly need to add rosin flux.
Heat the joint and first apply a bit of solder to the point where the parts meet and touch the iron tip. As it flows out it will help transfer more heat to the rail and joint. When the joint is hot you can touch solder to the joint not the iron and it will melt.
When the joint cools it should look smooth. If the surface of the solder looks dull or like a prune it is a cold joint and needs to be reheated. If not, give the wire a tug and move to the next one.
As for equipment, get a good grade of rosin-core solder. I don’t know if RoHS mandate has hit the island. We still use 60/40 lead/tin solder, or perhaps eutectic 63/37 blend. If lead is outlawed, frankly I don’t know what the easy RoHS somewhat-equivalent would be, but others might.
As for the iron, you will want 45 watts or more to solder code 100-125 rail. A 25 watt pencil is great for decoder installation but won’t deliver the heat fast enough to raise the rail to a soldering temperature.
We tend to use temperature-regulated soldering irons for our rail work. Either the tip self-regulates at a given temperature (650F or 750F being common for work on the layout) or there is a temp adjust dial instead of a wattage adjust dial. Tips stay clean longer when cooler but they transfer heat faster when hotter. We commonly use a 50 watt Weller WES 50 or WES 51 with a large, fat chisel tip. It can make a fine birthday present.
All the best. Please keep us updated with your progress and work!