For now, you can really work on good oral hygiene– brushing 2-3 times per day, rinsing with an anti-bacterial mouth rinse (Listerine or Crest Pro Health) and a fluoride rinse (ACT). Keeping it clean and knocking down the bacteria level are important to slow growth of the cavity.
I would strongly encourage getting the cavity filled as soon as you can afford it, before the cavity gets into the nerve and causes a toothache. Then your options become root canal therapy or extraction of the tooth. Neither one is as simple or affordable as a routine filling.