What data did the earliest HUDs display (for fighter jets)?
Reflector Sight:

The Me 262 Schwalbe, introduced in 1944 as the world's first operational jet fighter aircraft, did not have a full-blown HUD. Instead, like many fighters of WWII, it had a reflector sight which used lenses and reflecting glass to display a reticle. This sort of thing [all of the bright yellow lines you see in the image are fixed together. Although you only really needed a dot and a circle (like the Spitfire), some pilots liked the extra lines to help line up targets that were not dead-ahead. They were favored by pilots over iron-sights because the pilot was not required to position his head in order to see where the guns would fire.

Simple HUD

The MiG-21, introduced in 1959, used a combination of reflector sight and HUD. Though it didn't display the altitude, speed, heading, or attitude in a way that modern fighter pilots may take for granted, it had some ability to track targets with radar and to display predicted target positions for gun or missile use. Here is a guide on how to aim your guns using its sight. Also, explanations of the displays. The following image was posted on a forum thread related to the MiG-23, which came after the MiG-21, but this image looks much like those from the MiG-21 manual. A different image from another author shows you how simple the display could get. Finally, an F-16 in the sights of a MiG-21. You can tell this is a MiG because of the distinctive pitot tube.

Full HUD

The F-14 first flew in 1970. Its HUD was equipped with an artificial horizon, altitude, speed, heading, gun data, missile data, and much more. This is from the F/A-18, which first flew almost a decade later.