Appearance

Luffy is renowned for his trademark straw hat (from which he gets his nickname "Straw Hat Luffy") which was lent to him when he was young by the legendary pirate captain, "Red-Haired" Shanks, who in turn received it from Gol D. Roger. His outfit somewhat mimics Shanks' own outfit in that he wears short trousers (although his are much shorter than Shanks') along with sandals and a vest. Luffy also has a scar with two stitches underneath his left eye (he got it in Chapter 1) and short black hair. He was severely wounded by Akainu in the Whitebeard War, leaving a large X-shaped scar on his chest. Appearing to be scrawny underneath his shirt, he has a surprisingly well built physique. Pre-timeskip he was very short, dwarfed by most characters in the series.
Luffy, for the most part, has the same outfit throughout most of the series, as opposed to the rest of the crew, who change frequently. Luffy may wear a different outfit to suit the climate of an island (like a hooded parka on Drum Island or desert robes in Alabasta), but will switch back to his trademark vest and shorts after the adventures on that island are over.
Though Luffy prominently wears a red vest and blue shorts, he will wear his usual outfit in different colors. For example, in Enies Lobby he wore black shorts because they had pockets in which he could carry meat, on Thriller Bark he wore an orange vest and red shorts, and on Sabaody Archipelago he wore a blue vest and dark blue shorts. During the break-in at Impel Down Luffy was shown wearing a light blue vest with red shorts and after the fight between Magellan and Luffy, Inazuma gave him a yellow vest. He does, however, no matter where he is, wear his Straw Hat through absolutely everything, though at the start, the hat was prone to getting knocked off his head easily, forcing Luffy to either put it aside or have someone hold onto it during fights. He has since gotten a string strap for it which was sewed on by Nami, allowing the hat to hang from his neck during fights.
Luffy has been known to sport other miscellaneous items on his person from time to time. During the Skypeia Arc, he was seen wearing a black wristband around his left wrist with a white bangle over it, and after the battle against Moria in Thriller Bark, Luffy wore a decorated dark armband around his left bicep. This armband was later revealed to be a map that leads to Captain John's lost treasure, which Buggy sought for some time - leading to Luffy handing the armband in for Buggy in exchange for his services at helping him navigate through Impel Down.
During his return to Marineford to give a silent prayer, Luffy was briefly shown with four characters written on his right arm spelling 3D2Y, with the 3D crossed out to symbolize that the Straw Hat Pirates should meet up in 2 years (2Y) rather than the 3 days (3D) as planned after fleeing from Bartholomew Kuma.
After those two years, a few things have changed. Luffy wears a long-sleeved red cardigan with 4 buttons on, with a yellow sash tied around his waist, somewhat reminiscent of Gol D. Roger's outfit. He hasn't grown significantly taller, but is shown to be significantly more muscular due to his training. This can be seen in instances where his neck is slightly thicker, and his chest also having more definition. He also has a X shaped scar covering most of his chest, that was dealt to him by Admiral Akainu.