Since the dawn of time, students have struggled with how to start their papers.
Don't do this.
Write it out to get it out of your system if you must! The blank page is intimidating, and this can help you overcome that, but be ready to edit it out in your revisions.
"If you want to make an apple pie from scratch, you must first create the universe."
It'd drive you crazy if you just wanted a pie recipe and the author started out talking about how since the start of the universe, carbon molecules have been forming complex bonds and... You don't even especially want to know the origin of apple pie, right? Skip to the ingredient list already!
So, too, with your paper. Skip the universe, skip the evolution of the apple tree, skip describing the apple orchard, and start talking about your pie.
Your thesis is where you put forward your argument in a concise, declarative way. It is typically one sentence long and comes at the end of your introduction paragraph. You should only develop your thesis after you've started doing your research. You can have a thesis in mind as you start your research, of course, but be prepared to change it if you find it's unsupportable with the information available to you.
Thesis statements should be:
Your thesis statement should essentially give your reader a preview of what arguments you'll be presenting over the course of your paper.
If we've got the statement:
you know you can expect that
a) I'm arguing that oatmeal raisin cookies rock,
b) that I'm going to spend at least one paragraph discussing ingredients and their nutritional values,
c) another paragraph will discuss moist texture, and
d) another will discuss how consistent they are despite a variety of bakers.
Which ingredients? How healthy are they? Well, you'll have to read on to find out. But you know (that I'm arguing) that there are health benefits! And so on.
You want a variety of sentences in your paper -- some short and simple, occasionally a lengthier, more complex one. As ever, it's about balance. Too many short sentences is more choppy (and can feel over-simplified). Too many long compound sentences becomes a chore to slog through.
Don't mistake "formal, academic" writing for "overly-complicated." That is...don't make a special effort to sound Fancy. That's actually counterproductive to your true goal, which is simply to communicate information to your audience. (Sometimes that means being Fancy because you really need the precise nuance of a specific word...but again, don't make a special effort of that.)
Generally speaking, while you may have a lengthy paragraph, you probably don't want one to be longer than a page.
Start a new paragraph when you change topics. The obvious variant of this is switching between cookie ingredients to the moist texture (you read the thesis example, right? That's where this cookie nonsense is coming from!). However, what if we have a lot to say about ingredients? Rather than cram it all into a single paragraph, we can start breaking it up into a paragraph about oatmeal, then another about raisins, and another about cinnamon.
The sentence isn't over until you've given the citation (Source). But what if you use that same source for several sentences? (Source) That starts looking a bit redundant (Source). It's not very fun to read, either (Source).
You may compromise on documentation in favor of readability. If several sentences all use the same source, wait until the last one to add the parenthetical citation.
Let's say you have several sentences using a source, and then the first couple sentences of the next paragraph use that same source: how many parenthetical citations should there be? Two -- one to wrap up the first paragraph, and again for the second paragraph.
The usual advice: restate what you wrote.
However, that's a not quite all there is. Your conclusion should be more than just a rote summary (though you might do a smidge of that). The conclusion should be your big picture what it all means landing. If you can't picture dropping the mic at the end your paper... it's probably not strong enough.
You're not making an explicit call to action: you're analyzing a situation and providing evidence to explain what's going on.
You're also not bossing around your reader: "You should try an oatmeal raisin cookie."
This can be a challenging balance to strike. You're writing in a stiffer tone than how you would talk to your friends or even respond to a reflection paper, perhaps. But this isn't the same as hitting up the thesaurus for all the words and writing labyrinthine sentences!
Read one of your scholarly sources from your research with an eye towards its tone. While you might encounter some specific jargon, they usually aren't a showcase of every ~fancy~ word that the dictionary can produce.