Q. How do you know which Gods to worship?
A. This can be a difficult concept to grasp for anyone used to a simple, monotheistic faith. There is no confusion or questioning when there is only one God to deal with. But what about when there are hundreds to choose from?
First of all, don't worry about it. If you find that the numerous pantheons are too complex to work with at first, there is nothing wrong with simply worshipping the God and the Goddess as unnamed Deities. Many Wiccans believe that "All Gods are one God, and All Goddesses are one Goddess" meaning that all the various Deities really just boil down to a single Being anyway.
If you prefer a more individualistic approach, you will want to work with a specific God or Goddess. So how do you choose? One way is to spend some time in meditation, and ask that a Deity make themselves known to you.
Then watch your life carefully for any signs that a God or Goddess is trying to get your attention. This passive approach works for some, but not for everyone. Remember, this is a very personal religion you've chosen and many experiences are different from one person to the next. If there doesn't seem to be any signs, then it's up to you to take the matters into your own hands.
Take some time and research the various pantheons (groups of Gods, such as Greek or Egyptian) and get a feel for them. Choose a God or Goddess that you feel would suit you. Don't choose based on some immediate need either. You're going to be developing a long-term relationship here. Learn everything you can about that God or Goddess, and then approach them. Invite them to your rituals, add items to your altar that reflect their qualities, spend more time in meditation looking to connect with them.