Luci has summed it up rather well, although you may find it easier by doing it in dollars so for every $10.00 you spend, give a $2.00 tip, where justified.
Except for porterage where it's usually accepted at $1.00 per bag, unless extremely heavy, then $2.00 per bag.
You only need to tip, where you have received a "Service"some establishments have a tip jar, just for the fun of it, for example, gas stations, small shops, you are not obliged to tip here, but if you have any loose change, and feel it's justified then if you wish put your lose change into the jar.
Tipping is no different in the US, than it is in the UK, although there seems to be a big issue regarding tipping in the US, which frightens most UK visitors.
If you are used to tipping in the UK, then it will not be uncommon who and when to tip in the US.
Most Americans tip out of habit, because it's bread into them, unlike us here in the UK.
That being said, if the UK Customer Service industry followed that offered by US standards, many more UK citizens would be willing to tip.
Now off my soapbox.
Trust the above helps.
Driver02