Newgrounds is one of many websites where you can upload your Construct creations. They'll need to be exported using the Web (HTML5) option, and the resulting zip file can be submitted to Newgrounds. The whole process is nice and straightforward, and there are plenty of resources to help you out, so let's go through it!
Step One: Export your Construct Project
Newgrounds accepts games in the SWF or HTML5 (zip) format, seeing as the former doesn't apply here, that leaves you with one export option in Construct - Web/HTML5. You'll end up with a zip file, and this is what you'll be uploading to Newgrounds. In your zip file, there should be a file named index.html - this needs to remain in the root of the zip otherwise Newgrounds will throw a 403 error at you!
Step Two: Upload to Newgrounds
Firstly, you're going to want to make sure you're logged in to Newgrounds, if you've not created an account on the site, do so now. On the Newgrounds homepage, to the left of the search bar, you'll see a button that says 'Upload your Creations'.
Clicking this button will take you to a new screen which allows you to choose the type of media you're uploading. Obviously, in this case, you'll want to click 'GAME', and this will bring up a page where you can name your project. Type in your chosen title, and click next.
Now you should see a page where you can upload the game file and add in all the data necessary for your game, including a cover image, descriptions and tags.
You'll also need to rate your game in terms of adult themes such as nudity, violence and explicit content (is there none, some or lots?) You can then either preview your game or publish it.
Step Three: Promote and Play!
Newgrounds has specific rules surrounding new submissions, and if your game doesn't meet their terms, or doesn't gain a high enough score when it's under review, it will be removed.
From the Newgrounds site:
Game and movie submissions appear immediately on the site and go through a judgment phase where users vote on them. If a sufficient score is not achieved by the end of judgment, the submission will be blammed, aka deleted. A submission may pass judgment and still be removed by moderators if it breaks the rules stated on the submission form.
There's more information on the Newgrounds wiki.
So, to keep your game on Newgrounds, you'll need people to play and review it! Get out there and tell people about your game, and if they have feedback for you then pay attention, after all, your game may well be at the mercy of Newgrounds players!
And that about covers uploading to Newgrounds. It's a relatively simple process, but there's plenty of information that you'll need to fill in and get right to make sure your game makes it through the review process. Be sure to familiarise yourself with Newgrounds terms and policies before you post.
Good luck with your game!
If you want to know more about publishing to the web in general, Ashley has a rather handy tutorial for you.