There are hundreds of threads about this already. Searching for and reading them would be your best bet.
Otherwise, publishers/ developers each have their own set of rules and what can or can't be used in a lets play. There are a few websites that try to keep up to date rules for each (a quick Google search should find you them), but your best bet would be to go to each game's website and see what they say themselves before you do anything.
In addition to what the publisher states, a basic rule of thumb would be to mute any in-game music and avoid showing cut scenes.