Я иду в магазин = иду внутрь магазина (делать покупки и т.д.). Предлог "в" означает цель движения, куда необходимо попасть. Я иду до магазина = иду, пока не окажусь рядом с магазином (внутрь не буду заходить). Предлог "до" означает конечную точку движения, дальше которой движение не происходит.

The 2nd phrase is often used by the Ukranian speakers to say they're going to a shop. But Russian speakers use the 1st phrase to express the same idea. The 2nd phrase in Russian language has a different meaning which had already been explained by the previous comments. In shirt, if you want to say that you're going to a shop, use the 1st phrase: Я иду в магазин.

Я иду в магазин - I'm going to the shop (to buy something) Я иду до магазина- I'm going towards shop. (and then I'll meet my friend there, or will do something else , but near the shop. It mean that you aren'r going to come into the shop)

It is special for you :) http://www.italki.com/entry/331378