I used to own a Reddit account but I haven't checked it for long time. First you have to register your account (I can't remember though if you should verify your account through e-mail but I guess you know how to figure that out)
Once you've registered, sign in.
Then at the right-hand corner of your screen you see the "Submit A New Link" tab
This will direct you to a page. There are two tabs: "link" and "text". If you want to make a post and add a URL link choose "link" tab. But if you're going to post a text without adding a link choose "text" tab.
Under the link tab, there are three fields:
1. Title (where you enter the title of your post, of course)
2. URL (where you post the link)
3. Choose A Subreddit (I think these are tags -- but only one tag or subreddit is allowed)
A box of captcha code appears, which you should type in. For verification purposes, I think, to make sure you're not spamming or something.
Then click "submit" button.
I hope that will help :)