The Federal Communications Commission is expected today to take its first major step toward increasing competition in the broadcasting industry since President Reagan assumed office.

Commission sources predict the commission will give its final approval to a brand-new television service that could result in hundreds, and perhaps thousands, of new low-power television stations that would offer service to a small geographic area.

Under a proposal by the commission's staff, operators of this service would face few regulatory restraints now imposed on existing television stations. Additionally, any company that wanted to offer the new service--including television networks and cable television operators--would be free to apply for a government operating permit.

What's more, the application procedures would be streamlined radically to enable the commission to process the thousands of applications now pending for this service.

Under the staff proposal, the traditional procedures of granting licenses through a hearing before an administrative law judge would be eliminated. In its place, the FCC staff would review competing applications and recommend the best applicant--with a preference going to minority-owned firms--and then wait for the commission to make the final decision.

Also today, the commission is expected to set off a sharply competitive battle in the AM-radio industry by refusing to act on industry pleas to select an AM-stereo radio system as the sole system for the nation.

Five companies with mutually exclusive systems are vying for the AM-stereo market. Eager to get the service to consumers as quickly as possible, the National Association of Broadcasters has urged the commission to select one system instead of letting the marketplace make the ultimate choice--a process that could last for years.

But the commission staff has concluded that the marketplace is where determination of the best system should be made.

"There is no reason for the government to be involved in this decision," one commission official said.

Another official noted that there is not enough distinction among the systems to make a clear choice. As a result, any FCC selection probably would be subject to years of court challenges, creating even more substantial delays in the service than would happen in a free market, the official contended.

If the commission adopts these two proposals, it will be the first time the commission has moved to open the way for new telecommunications technology under the chairmanship of Mark Fowler, a Reagan appointee.

Since it was proposed, the commission has been flooded with 6,000 applications to begin the low-power television service. Once it is approved, the FCC expects to receive several thousand more applications. As a result, commission officials say that any low-power service is at least months away because it will take that long to process the huge volume.

Under the program proposed by the commission, low-power stations would operate on any available VHF and UHF channels (channels 2 through 69) where service would not interefere with any existing full-powered television station.

Unlike the full-powered stations that operate over a 60-mile radius, these stations would offer service within a radius of 10 to 15 miles. Under the FCC's proposal, these stations would not have to maintain a studio, provide local shows or ascertain the needs of their communities to keep their licenses, as is required of the full-powered stations.