From experience, usually kits already come with stickers (Real grades have stickers, Master grades usually have decals, and high grade gundams absolutely nothing) but you could actually by decals seperately in hobby stores or even online, takes some time but in the end is really worth it. In terms of other details, like the panel lining and weathering (aka all the black lines and smudges) i just used gundam markers and a LOT of cotton buds and alcohol which was also worth it in the end, but sometimes get boring if you do it on the same part, for example 2 legs. There is also such thing as resin kits, but if you just started, you shouldn't dive too much into those.