I’m a noob please help

I currently am riding a single speed bike (boardman comp I think) and I really want to get into cycling more as a sport. My knowledge of bikes/biking really sucks so I was wondering if anyone could help me in my situation, like should I invest in new bike with gears, should I track the time I take to do rides, how often should I cycle.

I’ve had a look at Facebook market place for bikes but I’m not sure if what would be most suitable for me. (Idk if this is important)

Like right now I’m just cycling whenever for fun usually around 30 km I would guess, but I would appreciate any help and guidance.