As mentioned, Usenet Providers are your gateway to the content available on usenet. Previously, your ISP may have acted as a provider, but in recent years this has become less and less likely. Those that do offer a usenet service now often provide a throttled service or access to only certain groups.
In addition to price, you may want to consider how fast you can download from the provider in real life (a free trial is useful here), how long the retention is (i.e. will older uploads be retained on the servers) and you may have personal concerns regarding privacy. Some providers bundle a VPN service in with their package.
To get you started, I've compiled a list of reputable providers that can offer you a free trial