The best part about Salamanders (fluffwise):
They are one of the original Legions, which means they have a history stretching back to the formation of the Imperium.
Like the Space Wolves, they live on a Death World which shows that they are some tough SoBs, who willingly choose to live on a hellhole like Nocturne.
They are very committed to history: the wandering Chapter hero who takes on Vulkan's name is seeking to discover the greatest of their progenitor's creations, but aside from that they have a wealth of ancient equipment. They are known to have the oldest Rhino in the Imperium, for example. They also keep their equipment in good condition. I like to reflect this in my Salamanders by modeling them with much older pieces of equipment, mostly older Marks of armor, but with older bolters or melee weapons when I can find them.
They espouse a doctrine of protection of the weak. It is not enough for them to merely fight for the glory of the Emperor, they seek to shield citizens of the Imperium from the dangers of the galaxy. Unlike many Space Marine Chapters, they often live within the society of Nocturne. For some of them, that leads to some internal conflict, due to living among and protecting a population that they can never truly be a part of.
Thematically, the Salamanders frequently see themes of volcanoes, the forge, and fire. Their primarch, Vulkan, shares his name with the Roman version of Hephaestus, the Greek god of fires, the forge, and volcanoes. (Coincidentally...

) Utilizing that in your modeling can give you a greater feeling of depth and meaning for your army. (It certainly does for me.)
The big event of the
40k backstory, the Horus Heresy, unfortunately doesn't see a lot of involvement from the XVIII Legion. The Salamanders were dealt a crippling blow, losing the majority of their Legion at Isstvan V, after which they spent the majority of the Heresy recovering. As a side note, the popular story is that Vulkan died in the infamous Dropsite Massacre, but no body was ever recovered, and at least one older source refers to Vulkan arguing strenuously against the implementation of the Codex, which occurred long after the battle he is supposed to have died in. This is another one of those situations in
40k where you will find multiple different accounts of the same event.
In any case, I hope that helps, good luck starting your own Salamanders army!