When you write in lernu!, we recommend that you use the "x-convention" (for example, "sx" for "ŝ"). That method is the clearest, and it is supported by this website in the instant messenger and the exercises. To type letters with "supersigns", it's necessary to install a program like Ek! or Tajpi (for Windows) (Ek! is available in Esperanto only; Tajpi is available in Esperanto and in English). For Linux users, you can follow the procedure describe here.