Here is the good news; measuring the bike frame of your current bicycle is not going to be difficult. Doing so is going to determine the size of the bottom bracket that you need. All you will need is a measuring tape to get the job done.
With that tape in hand, you will need to measure the inside of the bottom bracket shell that is in your frame. You can access a tutorial video if you are unsure where that is precise. Anyway, you will need the measurement to be in millimeters (and it will either be 68, 70 or 73 millimeters).