Xcode 9
If this is your first time to submit an app, I recommend going ahead and reading through the full Apple iTunes Connect documentation or reading one of the following tutorials:
However, those materials are cumbersome when you just want a quick reminder of the steps. My answer to that is below:
Step 1: Create a new app in iTunes Connect
Sign in to iTunes Connect and go to My Apps. Then click the "+" button and choose New App.
Then fill out the basic information for a new app. The app bundle id needs to be the same as the one you are using in your Xcode project. There is probably a better was to name the SKU, but I've never needed it and I just use the bundle id.
Click Create and then go on to Step 2.
Step 2: Archive your app in Xcode
Choose the Generic iOS Device from the active scheme menu.
Then go to Product > Archive.
You may have to wait a little while for Xcode to finish archiving your project. After that you will be shown a dialog with your archived project. You can select Upload to the App Store... and follow the prompts.
I sometimes have to repeat this step a few times because I forgot to include something. Besides the upload wait, it isn't a big deal. Just keep doing it until you don't get any more errors.
Step 3: Finish filling out the iTunes Connect info
Back in iTunes Connect you will need to complete all the required information and resources.
Just go through all the menu options and make sure that you have everything entered that needs to be.
Step 4: Submit
In iTunes Connect, under your app's Prepare for Submission section, click Submit for Review. That's it. Give it about a week to be accepted (or rejected), but it might be faster.