What do they generally pay the servers where you live? I've heard that outside of the US, most places pay them a reasonable hourly wage and the tips are something they earn above and beyond that.
Here in the US it's a lot different. Their tips *are* their wages - which is why your server would get pissed off at you if you only left a 10% tip or no tip at all. You should plan on 20% in the US, but definitely no less than 15%. And if you have problems with your meal, particularly if it's not the servers fault, don't take it out on them by not tipping them - they still waited on you.
Restaurants in the US take advantage of servers pretty badly and do a lot of illegal things too which they are not supposed to on top of it all. Servers in the US only get paid about $2.13 per hour in most places. For some stupid reason, restaurants lobbied to get exemptions from the minimum wage requirements, so they don't have to pay the full minimum wage with the understanding that the server "makes up the difference" with their tips. The restaurant is *supposed* to make up the difference if the sever doesn't earn more than minimum wage during a shift, but they never do. So tipping in the US isn't for when someone goes above and beyond and gives you exceptional service, it's for the fact that you got any service at all - it's their wages. Even if the server was having a bad night, it's especially crappy on your part to not tip them at all. They didn't come in to work that day to wait on you and others for free.
To make matters worse, restaurants engage in an illegal practice known as tip pooling - meaning they also underpay other members of their staff too, including bartenders, bussers, food runners, and more - and the servers are expected to give a portion of their tips to these employees as well at the end of their shift. Some restaurant owners are such mega peanuts that they actually keep all of the employees tips.
And if that wasn't bad enough, the servers have to claim how much they made in "tips" as wages at the end of each shift for tax purposes. If they claim anything less than 8% of their total sales for the day, they will likely get audited by the government. So if they honestly didn't make 8% in tips on a shift they have to claim they did and get taxed on it anyway.