The answer will vary depending on which tier you are playing, but all tiers should have some helpful resources for players to learn about the tier. For example, the
OU subforum has resources such as a
Viability Rankings (whats good in the tier),
Speed Tiers (For determining speed creep evs), and a
general guide to the OU subforum, and on PS theres the
OU room. Theres also the
SmogDex which contains an analysis on most viable Pokemon for each tier describing how to use the Pokemon, good movesets, potential checks and counters, and good teammates. If your looking for another tier, then you can try looking through the
Competitive Discussion Subforums which contains links to other subforums such as UU, Other Metagames, and Smogon Doubles.