The thesis should something sufficiently interesting, about which reasonable people could differ, and which can be defended. "In this paper, I will show that two plus two is four" is obviously true and not worth writing about. "In this paper, I will prove that Colorado Springs has a tropical climate" is impossible to prove. Don't set your sights either too high or too low.
Not everything you assume to be true is actually self-evidently true. A lot of what happens in philosophy is learning about one's own assumptions and how to defend those, or to get rid of the ones that are indefensible. You might think it's impossible for anyone to disagree with the statement that "There are no true liberals left in American politics; even Clinton and Gore are basically Republicans," but not everyone will be like-minded.
If, halfway through, you can't figure out what the sentence you're writing has to do with the thesis, stop and try to figure out how it links. If it doesn't at all, it probably doesn't need to be there. If it does but you need to explain why, add the explanation.
Transitions are important; they lead the reader through your argument. Transitional words, though, should only be used for actual transitions. Obvious problems are using words like "therefore" and "thus" followed by a statement which has nothing to do with what came before it.
Texts have to be interpreted, which means you need to quote passages to support your interpretation. The following are not appropriate uses of text as they stand. (Nor are the appropriate citations.)
Evidence for a claim can be of varying types. A lot of the work in evaluating arguments rests in deciding how to evaluate evidence.
Avoid useless generalities: "In today's modern complex society," "throughout time people have struggled with political questions," etc. They waste space and make you sound like someone who doesn't have enough to say. You're also probably not qualified to say things like "philosophers have always struggled with the question of " How many philosophers have you read?
Avoid making absolutist, overly general claims. Any argument whose conclusion is "therefore Plato is totally wrong" is probably a bad argument. Remember that most issues worth talking about are not simple.
Most issues worth talking about also aren't cut and dried: the "other side" probably does have something worth taking into consideration. The best reasoning is that which understands and anticipates opposing arguments, and takes them into account.
With respect to vocabulary, there are a number of points:
Always follow the guidelines for quotations and citations.
A coherent argument is difficult to sustain over many pages. I recommend having someone else read your paper for clarity, cohesiveness, etc., before you hand it in.
Proofreading your paper is not the same as running spell-check. Spell-check won't catch common mistakes like:
Do not use rhetorical questions or slang in your writing. They diminish the clarity of your paper by making the reader guess at your meaning.