These prepositions can be used in many contexts with different meanings. In the context you provided,
1) It just will be a little step (on) or (in) a long road of the life and full of lessons learned. - the word order and the choice of words in this sentence is a bit off. I would say: It will only be a small step taken ON the road of life, a road full of lessons learned.
2) I hope we preserve our friendship for or (to) the rest of our life. - in many contexts, FOR + duration. So, FOR the rest of our lives
3) Thanks to or (for) our relatives who are here today. - You thank someone, you give thanks TO someone. (there's a very good online dictionary, http://dictionary.cambridge.org, that also gives very good examples and there you can see what prepositions go with certain verbs, etc.)
4) Thanks for being a support for all of us. - this is correct
5) She will live forever in our memories. - also correct
6) We will put (on) or (in) practice for being the best every day. - put INTO practice, but this phrase does not go very well with the rest of the sentence. You could maybe say... We will try to be the best every day./ We will do out best every day./ We will put into practice all that we have learnt and try to be the best every day.
7) The bonds of friendship that we have made in class and in or (on) the halls of the high school. - You have 'in' the hall, usually. But I would rephrase this sentence and say - We cherish the bonds of friendship that we made both inside and outside of the classroom.
