Now when you go to http://www.pixiv.net/mypage.php you'll see your home page. The first thing you'll notice are some images in the middle of the page, under the heading "みんなの新着イラスト". This is everyone's newest artwork. Under these you'll see もっと見る, which means "show more", and you'll be able to page through them like on Danbooru.

On the right, you'll see the top 3 images for the day, links to #4 to #10, and the same for the top 10 images of the week. Under each of these is the top #100 list. On the left, you'll see your avatar, under this the list of people in your "MyPixiv", and under this, a list of all the people you've bookmarked. If you click on your avatar, you'll go to your profile, where you can change your avatar (画像の変更) and edit your profile.

On the search bar on the top right, you'll be able to search for specific tags (in Japanese). If you change the option from タグ to タイトル・キャプション, it will search in image captions rather than tags. You can click on 人気のタグ for a list of tags. ランダム brings up a random selection of images, instead of most recent.

On the top left you'll always see ホーム (Home), 友人を招待 (Invite a friend) and ログアウ (Logout).

When you click on an image, you'll have a medium size version of the image in the middle, which you can click again to open the full size, the artist's avatar on the left, with the following links under it:
プロフィールを見る - A link to their page/profile
お気に入りに追加 - Bookmark this user
マイピクに追加 - Add user to MyPixiv
When you try and bookmark a user, it will bring a dialog asking if you want everyone to be able to see this user in your bookmarked users list, and then a confirm button. If you try to add someone, you will have a box where you need to type a message to the person, followed by a confirm button.

Above the image will be the tags for it (blue links, with a red one to the right). Towards the top right, you can rate the image on a scale of 1 to 10. Pixiv keeps track of 閲覧数 (How many people viewed the image), 評価回数 (How many people voted), and 総合点 (Total score). To the left, you can see all the artist's tags. If you click on them, it will bring images by the current artist with those tags.

The button right above the image to the right, このイラストをブックマークする, lets you bookmark an image. To view all the images you've bookmarked, when you're on your home page, click on the button on top titled ブックマーク管理 (Usually the 4th one). Underneath the image theres a text box and button for adding comments.

Remember: You have to be logged in to see images or pages. You cannot hotlink them AFAIK. And don't forget to add artist tags when you upload the images here.

And apologies for the wall of text :)