In the comic books who was the first X-Man that Professor X recruited?
In the primary comics continuity (Earth-616), Scott Summers (aka: Cyclops) was the first recruited student which would become a member of the X-Men.
Per Marvel's website:
As a teenager, Scott came into the foster care of Jack Winters, a mutant criminal known as the Jack O'Diamonds. After Scott began to suffer from severe headaches he was sent to a specialist who discovered that lenses made of ruby quartz corrected the problem. Soon after, Scott's mutant power first erupted from his eyes as an uncontrollable blast of optic force. The blast demolished a crane, causing it to drop its payload toward a terrified crowd. Scott saved lives by obliterating the object with another blast, but the bystanders believed that he had tried to kill them and rallied into an angry mob. Scott fled, escaping on a freight train.
Winters sought to use Scott's newfound talent in his crimes, and physically abused the young boy when he initially refused. However, Scott's display of power had attracted the attention of the mutant telepath Professor Charles Xavier, who teamed up with F.B.I. agent Fred Duncan in their mutual attempt to find Scott. Scott was rescued from Winters' clutches and was enlisted by Xavier as the first member of the X-Men, a team of young mutants who trained to use their powers in the fight for human/mutant equality.
Within the storyline of the comics, the first "new" recruit would technically be Jean Grey (then called Marvel Girl), who joined the existing group of students/X-Men in the first issue of the X-Men comics.
Looking beyond the first issue of The X-Men, Calvin Rankin (Mimic) was the first official (albeit brief) new member of the team, though he was not recruited, as he sort of blackmailed his way onto the team in issue 27. He left a short while later.
The next member was Kevin Sydney (Changeling) who was recruited by Xavier as a "secret member" (known only to the Professor and Jean) in issue 40. He died in issue 42.
The first long-term new member was Lorna Dane (Polaris), who joined in issue 60 after being rescued from the Sentinels by the X-Men. Lorna later leaves the active team roster with her boyfriend Havok (Cyclops' brother), but remains an ally and on-call member. Xavier did not technically recruit her.
The label "first real new recruit by Xavier" would technically go to Alex Summers (Havok) who was actively recruited by Xavier in issue 65. He leaves active duty with Lorna Dane, but like her, remains an ally and on-call member, rejoining briefly over the years.