I usually eyeball my rice so I use the finger method which is,

Rinse and drain your rice in a sieve first

Add it to the pot and level it off

Put your index finger on top of the rice and add cold water till it touches your first knuckle

Bring to a boil, reduce heat to low and cover until cooked

You can always buy a rice cooker but I think it's good to learn how to cook without specific instruments, it also cuts down clutter in the kitchen.