What features of organisms that can be used to build a phylogenetic tree?

Basically the reason we make phylogenetic trees is to figure out which species are most closely related to one another, so it's quite tricky - that's what Taxonomists do, they figure out what is related to what.

One way is to use morphological features - what they look like, basically. We can tell that a housecat is more closely related to a lion than a dog because of the teeth, the bone structure, the snout - things like that. On the other hand, a dog and cat are more closely related to one another than, say, to a fish - which we can tell because they both have four legs and are rather less scaly.

Another way is to look at the DNA, which helps determine how long ago the common ancestor of both species lived, which is sometimes more accurate than morphology - some traits can look completely different even if it's using the same alleles, so what we might have thought were two completely unrelated species might actually be really closely related.
On the other hand, sometimes mutations happen faster than other times, which means that the DNA might not tell the whole truth - that's why we compare morphology and genetics to try figure out the truth.

There must be other ways, but idunno -shrugs shoulders-

But yeah. Hope that helps!

Good question.First of all let we know the definition of phylogeny.Phylogeny is the evolutionary history of an organism.Particularly those organisms belonging to same race,specie,family etc.Phylogeny can be brought out by studying the past evolution of an organism i.e;its ancestry.E.g;we say that humans and apes evolved from a common ancestor by studying their evolution.This helps us to construct a phylogenetic tree like we write like this.Australopithecus-zinjanthropus-pithecanthropus erectus-Neandertheal man.From neanderthals evolved men.This are several other ways to express phylogenetic relation.Coming to your question i can say that phylogeny can devolope by studying organs of diffrents organism.On the basis of analogus and homologus organs,one can predict the evolutionary relation of an organism or build a phylogenetic tree.