This will sound counter-intuitive at first, but do NOT start by buying a backpack. This is a common mistake that most people make. How can you decide upon the best backpack until you know what you will put in that backpack? I recommend first finding an old backpack or day pack that you are no longer using. If you do not have one then buy a decent backpack on sale or find one at a thrift store.
Next, start assembling your supplies. Try to keep the supplies down to necessities and look for supplies that can do double duty. Create a manifest of your supplies and later you will include this manifest in your kit. The manifest can make finding items in your kit faster and easier. Keep in mind that you may be in need and others might need to access the supplies in your pack to render aid to you. Here is my list of Bugout Bag supplies (details).
After assembling your items select a sturdy tactical pack or backpack (details). Your pack should have plenty of extra space for clothes and additional supplies. I tried to make my kit as modular as possible so that I can quickly move items from one kit to another. For example, first aid supplies are in a small waterproof that can be quickly moved from the Bugout Bag to my hiking kit when I go on hikes. This modular format helps eliminate duplication between kits. The basic MOLLE first aid kit can then be used in the vehicle kit, hiking kit, and Bugout Bag.