Start with either A Player Of Games or Use Of Weapons depending on whether you have an interest in Board Games or military/espionage, respectively
Then read the other one
Then read Excession
Then read the rest of the series in any order; because honestly there’s a considerable drop in quality other than those, but by that point you’re already so in love with the setting that “a mediocre Culture book” is still better than “a good book from another series”.
Absolutely. My general advice is: