If you have a specific article you're searching for, Summon is a good place to start. Summon lets you simultaneously search the Library's book collection and many full text journal articles. See the sidebar for tips on searching Summon.
If you're searching for articles on a topic, you'll probably want to search a database which is more focused on mathematics than Summon. Here are some starting points for finding mathematics journal articles or conference proceedings. Look for the UBC eLink icons to check for full text articles.