If you can sign up on pixiv, it doesn't always guarantee that you'll be able to use the rest of the site later. There are many components to pixiv, and for someone who doesn't speak Japanese, this website can be quite a challenge--almost not worth using. This is a guide to hopefully dissipate that uncertainty.
Alternatively, you can change pixiv into English. If you go to 設定, "settings" (the middle text in the upper left hand corner) and look down there is a dropdown box that says 日本語 (Japanese). Click that and move it to English. Then click the button at the bottom to save your changes. Ta-dah!
Assuming you're logged on to pixiv, when you first open the website you'll see the main page. Here's a translated version:
That's basically all you need to know on the main page. If you want to go into detail in some other section, you should do that on your own (or request it of me). Online translators help a lot.
If you wonder why the page is so many colors, blame Pixiv's new gradient background.
Images from Pixiv
Pixiv has three image sizes: small, medium, and large. Or, thumbnail, sample, and full.
First, click the thumbnail to open your image's main page. Then click the image to have the full version open in a new window. That's the one you want to save. You can also look at an image's properties (right click + properties on firefox) and take a direct link from there, recommended if you want to link someone else right to an image.
Top Images on Pixiv
Images used to be categorized by popular tags and broken up into top 50 in that manner as well as the top daily, monthly, and weekly, but recently that top tags page was removed. So to see the top images on pixiv, you may look at the top daily (デイリーランキング), weekly (ウィークリーランキング), and monthly (マンスリーランキング). It is possible to see up to 300 of the top, with the very favorites on the first page, and 50 images per page. The top three display as thumbnails on the right side of the main page.