There are different ways in which a company can secure funding to start its construction business. The first that might come to your mind is to obtain a small business loan from a bank or other lender.
One of the requirements a bank will have for you is to present your business plan, which must include your business milestones and goals to make your business successful. Another option is to find investors willing to fund your business in exchange for a part of your company's equity. Investors will want your business plan details so have this prepared ahead of time.