How to find a grave/death certificate with only a name?

VERY VERY few countries allow death certificates to be shown, they are civil registration documents and people have to purchase copies, you may find death indexes for some countries, which enables you the details required to purchase a copy of the death certificate, burial records ( church) are more widely available and there are several websites which show some.

BUT if you have no idea where in the World someone died and no idea when that death occurred unless the person had a very unusual name you will be hard pressed to find them.

try

familysearch

findagrave

and if you have an idea of the country at least then there are several other websites.

Names in isolation are not how people research their ancestry, that is how websites databases are set up, which is why so many people who only search online do not have their ancestry as many seem to think everything is online when reality is that if you put every single website together they only cover about 5% of records that are available

If it's a recent death, (in the last 20 years) you won't find them. There is no international database, mainly because most countries have privacy laws that prevent it. If it's an ancestor, sites like ancestry are good because they have a large database you could tap into to speed the search up. But, if it's a relatively recent death, then you won't have any luck.

IF the person died in the US in the last 50ish years (since about 1960), then the odds are that they are in the social security death index. Start here, and after putting in the name, come down the left side to where it says "collections" and filter to ssdi. Warning, if the name is entered different than you know or expect, it might not show up.

https://familysearch.org/search

Outside of the US, you are pretty much out of luck. There is nothing international.

Last option.. do you have the name of a close relative, who may have died? you can google in hopes for an obit, then try to contact other relatives who MAY have the answer.

Death Record Search Database : http://www.deathrecordsinfo.com/Info