Tickets can be purchased at any metro, RER or tramway station, and when boarding buses. In addition, you can purchase tickets at many newsstands or tabacs (tobacco sellers) around the city.
Something to keep in mind: when purchasing tickets from an automatic distributor in a Metro or RER station, only debit cards and coins are accepted in some stations-- if you have only bills you may need to purchase tickets from a vendor at the "Vente" (Sales) desk. Similarly, when boarding Paris buses, you'll need exact change.
You should also be aware that at metro stations with booths marked "Information", tickets are sold exclusively via self-service machines; the staff behind the window are there only to answer questions and provide information on schedules, routes, etc. Automated ticket vending machines are slated to completely replace the traditional system within the next few years. A helpful hint: you can change the interface language of the self-service ticket machines to English. This should make it easier to find the ticket/s you need, despite the machines' reputation for being a little less than user-friendly.