As pjmorse said, the top of the chain is the Registrar. Examples of registrars are GoDaddy, Markmonitor, Dreamhost, Eurodns, etc.
The Registrar is where you (usually) bought the domain. If you don't know who is the registrar, chances are you can detect it from a WHOIS call. Depending on the TLD, the registrar is normally listed in the Registrar field.
Be careful because if this is a gTLD, such as .COM, the registrar listed might not be your registrar but the accredited ICANN registrar used by your hosting company to register the domain. For example, you bought the domain from FooDomain Inc. who is registering .COM domains through Dynadot. In this case, have a look at the Technical contact or Owner.
Actually, you should be the owner of the domain (the Admin or Registrant) but it seems from your question that an hosting company is listed as owner. In this case, chances are that this hosting company is the one "you" used to register the domain and this is the one you first need to contact to recover your account.
Once you regain access to your account, whatever is the hosting company or the registrar, you should be able to change the nameservers associated to the domain using their control panel. They have control over the nameservers, directly or indirectly via the ICANN accredited registrar.
Change the nameserver to the record you want to use and it's done.
What nameserver you should use depends on you. A nameserver is responsible for resolving your domain hostnames into IPs. It means you should point it to an other server, hosting company or DNS provider that lets you manage your hostnames. If you don't have one there are several choices available such as DNSimple or Zerigo.