Wondering where to practice playing new hero’s?

I’ve Been playing for a few weeks now, and there are so many characters that I wanna try out but I feel guilty doing bad in games while trying to learn. Is there a specific lobby I should be in to just train and get experience? Half the characters aren’t available for me to try in training and show up as locked. I’ve tried mystery hero’s as a fall safe but it’s not exactly what I’m looking for.