The content of your paper will mostly be dependent on the structure, for the content is the filling-in of the outline dictated by the structure. However, the particular kinds of arguments and evidence you use will depend on the topic you are considering, and the particular claims you are making.
Arguments. While all philosophy papers should advance an argument, there are many different types of arguments that one can make. Thus, writing in philosophy can take many forms:
Advancing your own original argument is the basic task of providing a set of premises and demonstrating how these premises lead to your conclusion. The premises can be asserted on the basis of a number of different kinds of sources that constitute evidence that they are true.
Exploring the consequences of a particular hypothesis or position is the task of assuming that the hypothesis, or the set of statements comprising the position, is true, and showing what conclusions can be drawn from them, either alone, or in addition to some other set of premises you assert.
Reconstructing someone else's position, theory, or argument is the task of putting the other person's position, theory, or argument into your own words with as little excess verbiage as possible. In so doing, you should always abide by the principles of fairness and charity. The principle of fairness says that you should always paraphrase someone else in a way that is as close to his or her intentions as possible. This prevents the possibility of committing the Straw Man fallacy. The principle of charity says that, when you may be confused about the author's intentions, you should always interpret him or her in the best possible light. This means, for an argument for example, interpreting the premises and conclusion in such a way as to make the argument valid instead of invalid, or strong instead of weak. This kind of reconstruction is itself a kind of argument because you will need textual support to provide the evidence that your reconstruction is as fair as possible.
Evaluating someone else's argument consists, generally, in showing that his or her argument is good or bad. The goodness of an argument may depend on many things, but as a rule, sound arguments and cogent arguments are considered good, while valid but unsound, strong but uncogent, and weak arguments are all considered bad. In any case, you need to provide your own argument to show that the other person's argument fits some established criteria of what constitutes a good or bad argument. In general, if you disagree with the conclusion of someone else's argument, you have two options for evaluating it negatively: you can either show that the argument form is bad (i.e. show that it is a weak argument), or you can show that the premises are false (or you an do both).
Evidence. One thing that differentiates philosophy papers from other kinds of papers (English papers, scientific papers) is the wide variety of the kinds of evidence that are routinely used and accepted to support various different kinds of claims:
The kinds of evidence you use to support your thesis may vary according to the assignment and the topic, but some basic guidelines should be followed.
Relevance. The evidence you use should be directly and obviously related to the claim it is supposed to support. If it is either indirectly or not obviously related, you need to provide additional arguments why this evidence does indeed support your conclusion. We have already seen some fallacies you may commit if you use irrelevant evidence, like Ad hominem, appeal to the people, or appeal to ignorance.
Reasonableness. The evidence you use to support your claim should be such that any reasonable person would accept it, based not on the information you have, but on the information you have given in your paper. This means that, in general, if your evidence is controversial or not obviously true given the information you have offered, you will have to provide additional reasons to accept it.
Sufficiency. The evidence you use to support your claim must be sufficient to support it. That is, you must have enough relevant, reasonable evidence to persuade your reader. This means that, for example, you may not make sweeping generalizations about what all philosophers have been concerned about throughout the course of history if you have only read a handful of philosophy texts.
Definitions. As I said above, philosophers are generally very concerned with definitions, and especially concerned with being clear and consistent. This means that if you are going to use or consider important philosophical terms or concepts, you need to define them as clearly as possible. However, a conventional dictionary is usually not up to the task of providing adequate definitions of technical philosophical terms. There are many reasons for this:
Citations. Different sources may be used for different purposes, but all should be documented clearly and correctly. Many of the kinds of evidence you will use to support a claim involve sources that you will need to document.