This is a mouse game; just left-click on anything that has a label. Your character will decide whether you're going to Look, Use, or Take the item. You don't get to choose what Sam does with the things you click on. If he picks it up, you're going to need it eventually; if he doesn't, you're not. Simple as that.

To use an item, left-click on inventory (the cardboard box in the lower left corner of the screen) and click on the item you want, then click it on the item in the environment you want to use it on. Right-click to put the thing away, or left-click it on the box if you want.

To talk to people, click on them; if you have a picture of Max in the dialogue choices, you can switch over and let him handle the interaction (with a due sense of dread, of course).

To skip a bit of dialogue, right-click.