Well the first thing to do is look at the requirements of your chosen lab. Depending on their systems you may not have to do any resizing yourself, or you may have to do all of the work, but you need to find out first. Big images are usually viewed form a longer distance than small prints, so don't need such high resolutions, unless you plan on hanging it, and allowing viewers the chance to walk right up to them. In that situation they will try viewing from nose distance. So it can be useful to hand the pictue where the viweer cannot walk right up to it.
If you really have no idea about preparing images for print at sizes bigger than 6×4 then starting with some smaller prints might be useful. If I were going to make a print as big as you are suggesting I would prepare my image as directed by the lab, and then make a print of a say 16×12 section of it to actually see what the quality will look like, you do have to make the print on the same paper etc. I suggest doing the crops at 16×12 so that you have a large enough section to view from a suitable distance for the large print. I suggest 16×12 as they can actually be quite cheap from some labs. My lab of choice only charges £1.15 for a 16×12 print for example.
Once you have done your proofs then you will know that you can send in the file for the big print safe in the knowledge of what you are going to get back. But first find your lab, and find out their requirements.