Hi Britta,
I am a Christian and my understanding of prayer is as follows;
Praying in its simplest form is having a conversation with God, it goes both ways- we talk to and listen to God and he speaks back.
In terms of praying to a saint, it's like praying with a friend, in much the same way as people may gather together to pray in a group, praying with a saint is praying with someone who we believe is with God, as people who follow Christ have eternal life, therefore saints are alive in the 'afterlife' for want of a better phrase. As a result, they can join in prayers, joining in the conversation with God, and asking on the praying persons behalf. (Not all denominations agree that you can pray with/ to saints)
Praying 'for' something: it's an element of the conversation with God, and He may or may not answer the prayers. It's not because of the amount of faith, or because enough people asked. With regards to someone getting well, it's a tricky one as its evident that not everyone will be healed through prayer. I believe that God sometimes heals through miracles and I have seen this happen, but equally he heals through modern medicine and scientific research, and sometimes the 'healing' comes through death. Christian theology teaches that in heaven there is no sickness or pain, therefore being made well doesn't always happen in this life.
Rituals: I think are for both the worshipper and for God. I think God delights in ritual as he made us sensory creatures, who respond to atmosphere, to music, to silence, to rhythms of worship. God doesn't need the ritual, as he is God, but I think he appreciates that it can help humanity to have a visual/ physical expression of worship. So yes, they are partly to get the worshipper into the right frame of mind, but are not necessary for Christian worship.
As for why I pray? For the same reason that I talk to my husband or my friends or family, because I love God and want to share with him my thoughts, feelings and frustrations, because it's a two way relationship, because I know he listens and responds, and because he speaks to me.
[disclaimer: I say 'he' in relation to God as that is a common term, not to denote God's maleness but to avoid saying 'it' or 'he/she')
Sorry, that was a long response! Hope it helps :)