Organic chemistry is not a game of memorizing 50 disconnected reactions; it is governed by simple principles of electrostatics: negative charges attack positive charges.
Electrophiles vs. Nucleophiles
A nucleophile possesses a region of high electron density (such as an unshared lone pair on nitrogen or oxygen, or a delta-negative charge). An electrophile contains an electron-deficient center (such as a carbonyl carbon with a partial positive charge caused by oxygen pulling electron density away).
Carbocation Stability Order
Tertiary carbocations > Secondary > Primary > Methyl. Why? Alkyl groups are electron-donating via inductive effects (+I) and hyperconjugation, stabilizing the adjacent positive charge.