GoKawiilA good first step when analyzing your collection is to consider its rarity. You can get a general sense of this with two factors: how old the game is, and the number of copies in circulation. Releases for older-but-not-ancient consoles like the Xbox 360 could still hold some value, but likely not as much as something for an early console like the Atari 2600, as the latter has been out of print for longer. It's not an absolute rule, but chances are your older games are worth the most.
Which console the game is for can have an impact on its overall rarity, too. GameCube games are particularly expensive, for example, and tend to fetch a higher price than PlayStation 2 titles despite being from the same era. This is because the GameCube itself sold fewer units than the PlayStation 2 by a significant margin, so its games also tended to receive fewer prints. Titles that are special editions, regional variants or from other obscure sources are also likely to be worth more. An extreme example is something like Nintendo World Championship Gold for the NES, a prize doled out by Nintendo Power magazine to just 26 winners, which easily fetches over $100,000 today.
However, even the rarest items are impacted in value by their overall condition. Games that are CIB (short for "complete in box," meaning all auxiliary items like manuals are present alongside the packaging itself) are worth much more than loose games, and even more so if they're still sealed. There are other clear visual cues that help determine baseline value: if a disc is scratched or cartridge art is highly degraded, it's not worth as much.