Brahman or God is the ultimate Consciousness or God element that sustains, pervades and supports the entire universe as well as all beings. However, since the human intellect cannot conceive of this Divine Being several symbols have been given to us which represent Brahman. Just as a wedding ring, sindoor or mangal sutra signify the sacred marriage. A national flag represents the nation etc.
The various gods in the Hindu religion are symbols of Brahman. They help the mind focus on that which is beyond the world. Every devotee understands that someday s/he must go beyond the idol to the ideal.
A pole vaulter uses the pole to lift himself to a certain height. He then discards the pole and rolls over to the other side. Similarly, every spiritual aspirant needs a symbol of Brahman, God, that he can relate to and identify with. The symbol helps him to rise to a spiritual height after which he discards the idol to become Brahman. So you need to choose a symbol that appeals to you, commonly referred to as ishta devata or preferred god. Like Krishna, Rama, Hanuman etc. Worship the symbol till you no longer need it, till you are able to focus on Brahman without the need of an idol. Emotional people need a form as a symbol. Intellectuals need subtler symbols like a chant or the sound of Om etc.
The truth is that everything is Brahman. But you do not experience that now. Until you do, you need to worship. Just as – you know you are Brahman and the food you eat is also Brahman. But you need to eat food until you get to Realisation!