LiveJournal never, ever deletes old entries. You can keep your old entries as long as you want.
You can read 400 of your old entries by clicking the "previous" links in your journal. But this can be a slow process when you've got a lot of entries and you're looking for one specific entry, and it won't help you find entry #401.
Where did entry #401 go if it wasn't deleted? You can find it in your archive. Your archive, also called your Calendar, can be accessed from the main page of your journal. Click on your archive and you'll see something like this:
On this page, you'll be able to see the number of entries you've posted on any given day. The number of entries is a link that you can click to go right to those entries. So, if you know you posted an entry on a certain day, you can find it quickly by clicking on your archive, and then the day.
If your journal's been around for years, you'll also see a link to view the other years of your archive.
But what if you only know what month an entry was posted? Well, you could click on each day, but searching is a little easier if you're in the habit of giving your entries subjects. You can click on the "View Subjects" link that's near the month in question, and then it takes you to a page that looks like this:
You can see the date, time, subject line and number of comments for all the entries posted that month.
There are a few other ways to search through your old entries in the FAQ on searching.
Questions about using your Calendar? Please comment! If you have questions about other things, please see our community profile for ways that you can get help using your new journal.