Gnar is a hard champ to play, the best way to learn him is through experience, but mostly, use your Q to poke, stack, and slow for escapes. your W makes your extremely slippery and (Mega) the stun is wide enough to capture two or three enemies in a team fight. E can be used as an escape or engage although while your new, try to only use it as an escape, if you are mega, most of the time use it as engage. your ult boosts your speed(mini) and gives you a high damage burst, stun, and slow wide enough to catch a whole team in the right situation.
Basic combos:
1. Q ,AA,AA
2. AA,Q ,AA
3. AA, Q , E , AA, AA, AA
4. (Mega) E , Q , W , U , AA,
5. E , Q , AA
(Modified version of #4 (E , U , Q , W is used for throwing enemies into your tower, team fighting, and throwing enemies out of tower.)

To properly play Gnar you need to play like a ADC while mini and a tank while mega, the key is the mental switch when you change, and don't forget to keep an eye on your rage bar.