The Atari video game, which can simulate a jet fighter or a racing car, is now going to be able to play computer.

Atari Inc. announced yesterday that it will begin selling later this year a typewriter-like keyboard to be attached to its popular 2600 model basic game unit that will allow users to transform the game into a small personal computer.

The price of the accessory--which with Pac-Man-like simplicity has been named "My First Computer"--will be less than $90. The Warner Communications Inc. subsidiary also said it will market a line of software to go with the keyboard, and it has plans to offer a keyboard attachment to its more powerful 5600 model video game.

With the keyboard and software, users will be able to perform a variety of simple financial, bookkeeping and educational tasks. Atari also will offer a new line of sophisticated games made possible by the keyboard attachment. Older game cartridges will still be usable with the keyboard attached.

The keyboard will give Atari a product to compete against the less-powerful Timex-Sinclair home computer, which retails for under $100. Mattel Inc. and Coleco Industries Inc., whose video games compete against Atari's, also plan keyboard attachments for their basic home video-game units.

"Basically, it's a defensive step," Lee Isgur, an analyst at Paine Webber Mitchell Hutchins, said of Atari's announcement. Noting that Atari has lost market share in the past year as other makers have moved ahead of it with new models and accessories, he added, "I look at it as being the first step in 'Atari fights back and eventually wins.' "

Perhaps more significantly, however, the keyboard would provide a transitionary step to full-fledged home computers for the more than 10 million households that have the Atari video game. "If you want a computer now, for $90 you can make this into a computer," Isgur said.

Those who don't already own the Atari game unit, however, may want to look elsewhere for their first computer: The price tag for the basic Atari game and keyboard--somewhere over $250 in all--makes it more expensive than the Timex and the Commodore Vic-20 home computers, for instance. "If you don't have a computer already and you don't have an Atari 2600 VCS video game, then it's cheaper for you to go out and buy an Atari 400, Tandy 99/A computer or even a Radio Shack color computer," Isgur said.

My First Computer will have 8K random-access memory, or a capacity for 8,000 characters, with optional attachments providing additional memory up to 32K and access to peripherals such as printers, disc drives and telephone modems.