Switch/Button Pressed to make Arduino Start Certain Lines of Code

Hello all,

I need some advice on a project I am currently working on.
Some Background: I am building a device that mounts to a golf club and is controlled via Arduino. It send data about your swing to your smartphone using various sensors I have hooked up. Right now I am fairly far into the build and have everything working correctly, except I need a way to trigger the Arduino to start collecting data (Serial) and outputting it. I already have the bluetooth working with my phone and my phone reads the data but it is constant rather than when I push a button, per say. Any ideas on how I can hook a simple momentary button up to start reading from the sensors when I push it? Thanks in advance and I would appreciate other suggestions as to how I should go about this as well. XD

Does the Arduino have enough rom to store a swing worth of data? Can the Arduino automatically recognize a swing and send the data for that swing? That way you don't have to annoy the user with a button to press.

The data output is just numbers so storage most likely wouldn't be an issue although it depends on how many datapoints I collect because to make a smooth graph of the swing would probably require more than the little amount of storage on the arduino uno, although I'm not entirely sure. An automatic swing detection would be nice, and if you have an idea on how I could do that I would appreciate it. Maybe with the help of the accelerometer it could be done? Thanks, Loren.

Surely anything attached to the club is going to affect the heft and the swing of the club, so unless that exact mass is going to be attached to the club when playing, the trainee is going to undergo negative training?

It is certainly a possibility, but I am an avid golfer and while testing my device it seems the arduino and sensors don't collectively weigh enough to make a large difference. I mounted the arduino and perf board on the handle and ran the sensors to the club head so the least amount of weight would be at the head. This adds up to little effect on the swing. I am only 16 and am doing this for school, not selling it on a massive scale so as long as it does it's job somewhat my goal will be accomplished. Right now I am mostly focused on a trigger to start data collection. Thanks, though.