The activities trekking and hiking are pretty similar because in both trekking and hiking you are walking trails.
However the difference between trekking and hiking is that trekking is more challenging and a more rigorous activity whereas hiking is a less challenging and less rigorous activity.
Both trekking and hiking are good activities to keep you healthy and fit but if you're starting out I would recommend starting with hiking first which is less challenging than trekking is.
Then after you get used to hiking you can start trekking and you'll likely enjoy both activities.
Trekking unlike hiking really tests a persons physical ability, endurance, and even your mental or psychological capacity.