>>107106593 (OP)
Unironically excel.
If you're not setting up a proper relational database to store maintenance and/or using real accounting software to track financial stuff, keep excel files logging everything. Every penny spent. The time you spend. How many days early/late your tenant's pay, the method that they pay, the transaction IDs.
>>107107058
Also this. Screen tenant applications very thoroughly. If so much as a hair stands up on the back of your neck, don't take them as tenants. There's always more people looking to rent. Keep your rents reasonable, don't dick your tenants around, and they'll stick with you for years.
>>107107119
This is also a good idea, but more relevant if you have multiple units. Renting a single unit can be done with a simple extra email and cheap secondary phone number (never give out your primary phone number).