"After the battle ends, the heroes made their way to Kami's lookout, renamed Dende's lookout because Dende became the Earth's guardian. Cell was gone but so was Goku, and after the Z Warriors attempted to wish him back after wishing all of Cell's victims back (this included Trunks), but failed because of Goku's decision to stay in Other World. Goku explained that he seems to attract evil people, so it would be better for the Earth to live in peace without him. He also said that Gohan surpassed him greatly now, so there would be no worries if another threat came."
He didn't want anymore evil to come to the earth, which is always attracted to him. He was looking out for the well being of the others.
And as for the time he didn't come after namek, he was training to get that instant transmission techinque ( or something along those lines), which comes in handy later on in the series.
Goku did fight cell first, but stopped because:
"Goku knew that if he continued the Earth would be destroyed, or Cell and himself would die fighting each other. Goku suddenly stopped fighting Cell and surrendered. He reasoned that only one person could defeat Cell. When Goku revealed his son Gohan as his pick to fight, the Z Fighters were furious (noticeably Vegeta and Piccolo) and Cell was amused."
And as for letting gohan battle cell, that's completely gohans fault goku died. Gohan was arrogant and was fooling around with cell instead of finishing him off right away, his power got to his head. If gohan had taken the fight seriously to begin with, goku would have never had to fight and therefor wouldn't have died. And maybe goku wanted gohan to fight first because he wanted to make him learn to be able to take care of himself. The kid can't always be saved by his father after all, he has to grow up himself.