The structure of an aqueous 1-n-decyl-3-methylimidazolium bromide solution and its vapor-liquid interface has been studied using molecular dynamics (MD) simulations. Starting from an isotropic solution, spontaneous self-assembly of cations into small micellar aggregates has been observed. The decyl chains are buried inside the micelle to avoid unfavorable interactions with water, leaving the polar headgroups exposed to water. The cation aggregation numbers, ranging from 15 to 24 compare favorably with experimental estimates. Results are presented for the organization of solvent around the cations. The structure of the aggregates as determined from the present MD simulations does not support the staircase model proposed on the basis of nuclear magnetic resonance studies on similar aqueous ionic-liquid solutions. The distribution of ions in bulk solutions and at an air/water interface is also discussed.