Looks like you have your Roth IRA with Vanguard. You see where it says "Buy and sell"? That's where you start. Choose "Buy Vanguard funds," and you'll be taken through a series of steps. You'll specify where the money is coming from (e.g., your bank account), which fund(s) you want to buy (e.g., a target retirement fund), which year you want the contribution recorded (until April 15, you can still contribute for 2014 OR you can contribute for 2015).
Seems like you need to look around the Vanguard site and get more familiar with your account. All the information you need is there, and you can always call customer service if you need help.