I've never uploaded art on pixiv myself, but I would assume it's like this:
1) Just under the pixiv banner, beside the ホーム (Home) tab, there should be the イラストの投稿 (File Contribution) tab. Click it.
2) Now it should bring you to a screen that shows three steps. The first is the Upload of File step (ファイルのアップロード). Upload your file and click the 情報入力 button at the bottom.
3) 情報入力 is when you write anything important that's related to your work, so basically a description. So here, you fill in the タイトル title, キャプション caption, タグ tag, 公開レベル (don't know what this is... something-level... I would just keep it what it is), 閲覧制限 restrictions (so if it's really NSFW or risque, then you might want to click R-18, otherwise it's 全年齢（一般）), イメージレスポンス image response (don't know what this is for), 使用ツール tools or what you used to create your work.
4) Once you are done, at the bottom, click the 完了 (finish) button to the right and click ok. The work should be uploaded now.
