Answer by taking a quote from the following article:

Iron Maiden are an English heavy metal band formed in Leyton, East London, in 1975 by bassist and primary songwriter Steve Harris. The band's discography has grown to thirty-eight albums, including sixteen studio albums, twelve live albums, four EPs, and seven compilations. Pioneers of the new wave of British heavy metal, Iron Maiden achieved initial success during the early 1980s. After several line-up changes, the band went on to release a series of UK and US platinum and gold albums, including 1982's The Number of the Beast, 1983's Piece of Mind, 1984's Powerslave, 1985's live release Live

On New Year's Eve 1978, Iron Maiden recorded a demo, consisting of four songs, at Spaceward Studios in Cambridge. Hoping the recording would help them secure more gigs, the band presented a copy to Neal Kay, then managing a heavy metal club called "Bandwagon Heavy Metal Soundhouse", located in Kingsbury Circle, northwest London. Upon hearing the tape, Kay began playing the demo regularly at the Bandwagon, and one of the songs, "Prowler", eventually went to No. 1 in the Soundhouse charts, which were published weekly in Sounds magazine. A copy was also acquired by Rod Smallwood, who soon became the band's manager, and, as Iron Maiden's popularity increased, they released the demo on their own record label as The Soundhouse Tapes, named after the club. Featuring only three tracks (one song, "Strange World", was excluded as the band were unsatisfied with its production) all five thousand copies were sold out within weeks.  In December 1979, the band secured a major record deal with EMI and asked Dave Murray's childhood friend Adrian Smith of Urchin to join the group as their second guitarist. Smith declined as he was busy with his own band, Urchin, so Iron Maiden hired guitarist Dennis Stratton instead. Shortly afterwards, Doug Sampson left due to health issues and was replaced by ex-Samson drummer Clive Burr at Stratton's suggestion on 26 December. Iron Maiden's first appearance on an album was on the Metal for Muthas compilation (released on 15 February 1980) with two early versions of "Sanctuary" and "Wrathchild". The release led to an ensuing tour which featured several other bands linked with the new wave of British heavy metal.  Iron Maiden's eponymous 1980 release, Iron Maiden, debuted at No. 4 in the UK Albums Chart. In addition to the title track (a live version of which would be one of the first music videos aired on MTV), the album includes other early favourites such as "Running Free", "Transylvania", "Phantom of the Opera", and "Sanctuary" - which was not on the original UK release but appeared on the US version and subsequent remasters. The band set out on a headline tour of the UK, before opening for Kiss on their 1980 Unmasked Tour's European leg as well as supporting Judas Priest on select dates. Iron Maiden also appeared, to much acclaim, at the Reading Festival 1980. They were second to top of the bill on the Saturday, with UFO headlining. After the Kiss tour, Dennis Stratton was dismissed from the band as a result of creative and personal differences, and was replaced by Adrian Smith in October 1980.  In 1981, Iron Maiden released their second album, entitled Killers. Containing many tracks written prior to their debut release, only two new songs were written for the record: "Prodigal Son" and "Murders in the Rue Morgue" (the latter's title was taken from the short story by Edgar Allan Poe). Unsatisfied with the production on their debut album, the band hired veteran producer Martin Birch, who would go on to work for Iron Maiden until his retirement in 1992. The record was followed by the band's first world tour, which included their debut performance in the United States, opening for Judas Priest at The Aladdin Casino, Las Vegas.

what was their first album?
Iron Maiden's eponymous 1980 release, Iron Maiden, debuted at No. 4 in the UK Albums Chart.