Mining: is there also "luck" factor ? I'm almost new in Bitcoin / crypto currency world. 

I know that "old" coin like Bitcoin and others (for old i mean 2-3-4 years), when you want to mine there is a difficulty variable to considerer. 

My question is: if i put my CPU (or my low computational GPU) to try to mine, can I except to be "lucky" and mine ..something ? 
My question is : is there also a "Lucky" factor to consider when you try to mine cryptocurrency, or .. it is only a cpu/gpu power matter ? 

Thanks
Yes there is a luck factor - mining is just probabilities, it is all about luck and probability. Technically every miner, no matter how big, has a chance of mining the next block. But more powerful miners have a higher chance, like they've bought more lotto tickets, than those who have only smaller mining rigs. This makes the probability of mining a block using a GPU so tiny that it's almost certainly never going to happen.

See this answer for more info: In the ASIC-age, is it worth starting mining Bitcoin at home?