It is usually simplest to set the level of your creatures to the same average (or majority) level as that of the PCs.
However, it is possible to adjust the difficulty level of your encounter by slightly varying this level for one or more of your creatures.
In this case, the level of the creatures should ideally be between one level lower and one level higher than the average (or majority) of your player characters' levels.
Creature Level = Average PC Level ± 1
Keep in mind that this rule is a recommendation, and the GM has the flexibility to adjust the levels of the creatures to fit the story or the ambiance of the campaign.