Anything other than known fin nippers (i.e. go for passive community fish), other males (obvious) and I'd research females as I have read that they will fight each other too.
Male bettas cant be kept with other males, they also cant be kept with females unless for breeding (but u gotta know what your doing) As for other tank mates, they can go quite well with corys and also nearly all plecs.
Never Tetras, for they tend to nip and either way, most of them can look like males.
The biggest no-no is a male with a male or a female with a male, unless when breeding, which they still can't be kept in for long.
Generally you could have just looked at the Betta threads or this.\ If you want to look further into it read that.