Don't know what path you are, but I suggest researching and getting some books for beginners to help you. As mentioned above meditation is really important. Energy work, chakras and circle casting would be a good idea to study. If you're wanting to just do magick, read some articles and forums explaining what magick is and what it can and can't do. Also, study colour association, crystals, herbs, and spell writing to start. If you wish to learn more about a pagan religion, I would learn more about the tools, rituals, beliefs, holy days and so on before casting a spell.
Regards to sacrifices to gods, research the deity and you'll find out what if favours. I don't follow Greek so I won't be of much help their. Also, meditate and call to them, you might connect with them and get your answer that way as well, but be careful, some deities are more accepting and kind while others might be angry you don't know simple things about them.