We all know that in the older format of Ygo we needed to tribute summon our strongest monster. (Dark Magician) But in our current format it's all about special summoning. So I will help you people who don't know how to tribute or who have no idea what stars are for.
When you look at a monster in your hand it will have a number of stars under its name. Those stars are to indicate how many monsters you have to tribute for it.
Monsters with 1-4 stars don't need a tribute sacrifice to summon. (Some have requirements you have to meet though, like Rare Metal Dragon)
Monsters with 5 or 6 stars only require you to tribute 1 monster to summon it.
These are mediocre monsters with decent attack or effects.
The most powerful monsters are 7 stars or higher. These monsters either need you to reach a certain requirement or tribute 2 monsters to summon them.
Rarely you will come across monsters that have amazing effects but you must tribute 3 monsters for. Also, the god cards are some of the common monsters that need 3 tribute sarcifices.