Most words ending with "-a" are female.
I realized that most words ending with "-a" are female. And even those not ending with "-a" tend to be male! :) Love this language, it's pretty simple. :)
I think the exceptions result from the etymology: words (ending with -a) that are originally Greek, are male in Spanish. The corresponding Greek words are male or neuter: ["το πρόβλημα" = "to próvlima" = el problema] ["το σύστημα" = "to sýstima" = el sistema] ["ο κομήτης" = "o komítis" = el cometa]