If you just want to get in "ball park" for your proper frame size you can start with the Lemond method where you take your inseam (measured with no shoes against a wall) and multiply by .67. This number will give you a frame size measured from the center of the bottom bracket to the top of the seat tube. Remember, this is just a starting point and the Lemond method tends to give too small of frame sizes for taller riders and some frame designs. Also different manufacturers use different points to measure their frames so you need to know that you are comparing apples to apples. You don't want to get caught out buying a bike with a sloping top tub thinking it's a 56cm when in reality it's measures closer to a standard 58cm frame. There is lots to think about!
If you don't know what you are doing you really need to find someone who can do this for you. Find a shop that utilizes a "fit" bike or at the minimum takes lots of measurements and is willing to guarantee their work. Most fittings for long distance riders take a couple visits to fine tune things. The other option is to become really educated, really fast.