You should not wear a belt and suspenders. It is one of the biggest fashion faux pas.
It Is considered fashionable to wear jeans and suspenders. They can be dressed up or down for a great casual fun look.
Suspenders should be snug not very tight. They should fit well and be form fitting per se, meaning they should not disrupt the ways your pants fit on your waist.
Suspenders are better than belts because they do not cinch your waist and ruin the look of your pants. They are more comfortable for long term wear as well.