To track your sessions, the minimum I would recommend is a heart‑rate monitor. This can be a chest strap connected to your phone, or simply the sensor built into your smartwatch.
There are also bike computers that can connect to different sensors: speed, cadence, heart‑rate strap, and possibly a power meter. But a power meter quickly increases the budget and isn’t necessary when starting out.
These bike computers can sync with your smartphone to record your sessions through their dedicated app. It’s an easy way to keep track of your progress without making things complicated.
Heart rate monitor example