Weakness
If your GM allows, you can begin with weaknesses. Talk with your GM about making weaknesses before proceeding. Weaknesses come in tiers much like special abilities, but are usually capped at tier 6. A GM may allow some exceptions to this cap, but this is the assumed maximum unless noted otherwise.
Note that weaknesses always entirely ignore Armor Rating and Resistance by nature of being weaknesses.
MINOR WEAKNESSES (TIER 1 or 2)
The weakness is either moderately difficult and rare or fairly easy to overcome but a constant inconvenience. Being forced to make saves to avoid tier 1 conditions that normally doesn't affect anyone else would be a common example. A weakness like this is more of an inconvenience like a compulsion or minor phobia.
Weakness to an effect type would mean that effect type deals additional (Tier)d6 in damage to you (dealing only this damage if it would've been 0 for most people). If there is a lot or a little, you can half (ex. partial, splash, minor, etc.) or double this damage (ex. full emersion, surrounded by it, ingested it, etc.).
MODERATE WEAKNESSES (TIER 3 or 4)
The weakness is either overwhelmingly difficult and rare or moderately difficult and common. A tier 1 weakness where you always have disadvantage on the checks or suffer tier 2 conditions would be a good example. A weakness like this is pretty detrimental like an extreme disorder, manic addiction, or psychosis.
MAJOR WEAKNESSES (TIER 5 or 6)
The weakness is either impossible to resist and rare or very difficult and common. A tier 1 weakness where you always have disadvantage on the checks AND suffer tier 2 conditions would be a good example. A weakness of this magnitude is not only crippling, but deadly and equivalent to sunlight for a vampire or kryptonite for a certain superhero.
LIMIT
A GM may set a limit to the number of tiers-worth of flaws you can have on any one ability and total weaknesses you can have (typically 3).