Position 2 - Mid/Core - The 2 position is typically played in the mid-lane, where they solo against the other mid-laner. These heroes can be carrys, but typically are heroes that except in solo situations.
Position 3 - Core/Offlane - The 3rd position is usually the off-lane, they play solo against 2 sometimes 3 heroes, their goal is to get levels, try and get farm, and try not to die and disrupt the enemy Carry as much as possible.
(Current Meta 9/19/17) -(Axe, Timbersaw, Faceless Void, Legion Commander, Bloodseeker.)
Position 4 - Support/Core - (My favorite position to play) The 4th position is a hybrid support/core/roaming hero. Their job is to gank the lanes whenever possible, help where needed, and try and get levels. This position typically need some farm for high-impact items (Blink, Aghs Scepter, Force Staff)
Position 5 - Support - The 5th position is the "poor support" - this posistion typically spends most of their income on observer wards, sentry wards, smokes, dust, TPs, and other items that support your teams visions, counter-vision, or gank potential.
I think the best way to get you into Dota2, is pick what position you like and start with that.. Hope this helps you, if you ever need more help learning, PM me and I can give you my steam and we could play some games together.