In short, roll a d20, add your bonus.

• using skills
• Full skill list and descriptions

If attempting to use a skill outside of combat, when not under pressure, you can “take 10” – treat your skill check as if you'd rolled a 10 on the dice. There is also “take 20” for skills with no negative consequences in the event of failure.

You can also aid another if you try a skill check on something your friend is doing: roll 10 or higher, they get a +2 bonus.

If attempting to perform a task that does not correlate well with a skill, use your ability modifier instead, e.g. adding strength mod to kick down a door, constitution for holding your breath, etc.