The need to reinforce walls or foundation before finishing an attic is something you will need to have evaluated by either your builder or an inspector. Most attics are not built to support the weight of a living space, especially if heavy items will be placed in the finished attic. Adding a bathroom in the attic (because of the weight of the fixtures), storing several very heavy items, using heavy materials when finishing the space (tile floor, hardwood floor), can also add more weight than the structure was meant to carry. If this is the case, you will need a contractor. It will be necessary to support the structure and add additional wall and foundation support to carry the extra weight.