Here are some examples:
* C'est un problème sérieux. (This is a serious problem.)
* Il a l'air sérieux. (He looks serious.)
* Prends ça au sérieux. (Take this seriously.)
You can also use grave in some contexts, especially when referring to something that is severe or important.
For example:
* C'est une situation grave. (This is a serious situation.)
* Une maladie grave. (A serious illness.)
The best choice will depend on the specific context.