For this you can use limit
select *
from scores
order by score desc
limit 10
If performance is important (when is it not ;-) look for an index on score.
Starting with version 8.4, you can also use the standard (SQL:2008) fetch first
select *
from scores
order by score desc
fetch first 10 rows only
As @Raphvanns pointed out, this will give you the first 10 rows literally. To remove duplicate values, you have to select distinct rows, e.g.
select distinct *
from scores
order by score desc
fetch first 10 rows only