While defensive options are something that can always frustrate players in traditional fighting games–given the aggressive nature of the games themselves–Super Smash Bros. Melee’s defensive options can be even more mentally taxing on a newer player. Instead of blocking in the traditional sense, Smash games require you to hold up a shield, which can be depleted and broken. To top it off, as a shield depletes, it can leave various hurtboxes on your character vulnerable to attack as they are uncovered by the receding shield.
There are ways to mitigate this, which are covered in this tutorial. It goes over shield tilting, which allows you to move your shield into a more vulnerable part of your body that you expect to be hit. It also discusses shield DI, which changes the direction of the pushback of a shielded move, as well as post-shielding options.
Source: SSBM Tutorials