I have a grandson who wants to build a LEGO building that looks like the high rise building I live in.
- Take pictures of your building, of as many sides as possible
- Take measurements or find blueprints of the building. Length, width, height, height between floors, size of balconies etc.
- Convert these dimensions to LEGO dimensions. For this, figure out the scale you want to build in first (e.g. 1:40, which is "minifig scale" - but check out this post for some caveats). You can use Sariel's calculator to find out what this means for how many bricks you need.
- For best results, design the whole building in LEGO Digital Designer (LDD) or LDraw first. This will save you from making unnecessary brick purchases before you have finalized your model
- Get a brick inventory from LDD/LDraw once you have your final model.
- Use a service like Bricklink or BrickOwl or Lego's own Pick-a-Brick to buy the bricks you need in the quantities necessary, after accounting for all the bricks you already have. Alternatively, you can look at other available sets to see if some of them carry the bricks you need in large quantities - this might be cheaper (you could even sell the not-needed bricks back via BrickLink etc.).