Friendly Gods

when calling on a deity, be respectful, all deities will be kind if you are kind to them. Do not summon, summoning is commanding their presence. Instead, ask, pray, invoke. The degree of kindness they show depends on:
1) an offering (not neccisary but it's nice)
2)the devotion you have shown towards them, do you pray to them everyday? Are they your patron?
3) their understanding of you, here's where medusa went wrong, she was going to be raped on the alter of Athena, who is a maiden, I'm not saying that Athena is not compassionate, I am saying that you must look at the circumstances, if she had gone to the temple of Persephone, who had a similar experience of being forced by a man, or demeter, persephone's mother, who missed persephone sorely, they would undoubted protect her.
4) are you worth their trouble? Have you proven your worth to them?
5) do you truly believe in them?
6) be sincere. Insincerity makes everything worse.
7) reseach this deity, are they known for being prideful, arrogant, a trickster or a tyrant?
8) worship, worship, worship! Worship is a great way to strengthen bonds with deities.
9) are you sure this is what you want? By doing the above, you are likely to be shown kindness, but that is only on the surface, if you're working with a trickster like Loki, do take into account that he has been tricking humans for centuries and have much more experience than you. So take care and be pedantic, working with them, you should not allow them to twist your wishes.

What do you mean by invoke? And do you know a website that I can go to, to read about the gods?

I'm sure you could look up the word "invoke" in a dictionary rather than expect us to do that work for you. But "invoke" is defined as "To call on (a higher power) for assistance, support, or inspiration:"

An excellent site to read about the Gods from a variety of different pantheons can be found at http://www.pantheon.org/

Invoking is what you should probably be doing instead of summoning. Summoning is demanding their presence, invoking is simply asking them to be there and to assist you. It's much more polite if you ask me.