Maintenance of tankless hot water systems is typically pretty simple. There is a myth that tankless water heaters require annual maintenance, manufacturers typically disagree with that. There aren’t many moving parts or things to go wrong on a tankless water heater.
If you have “hard water”, you will need to periodically flush out the minerals that have built up inside of your tankless water heater. This is a pretty straightforward descaling process which can typically be done by the homeowner or you may schedule visits from a professional. The process involves shutting down your heater and pumping a bit of vinegar through your water heater. The vinegar will dissolve any calcium that has built up inside of your unit. Instructions for descaling will likely be included with your tankless water heater manual.