First, go to My subscriptions. If you are already subscribed and it's too early to buy a following subscription then you'll see a summary of your current subscription with an advice that you're all up to date.
Otherwise you will have the opportunity to select a new subscription for the coming period for you and your family members. Make sure you choose your's (if applicable) before choosing ones for your family. This is because family member subscription may depend on what you have. ManageMyMatch only presents choices that are applicable for you - typically date and club distance dependent.
ManageMyMatch also applies all relevant discounts for you. Once you've chosen your subscription(s) you'll see the total amount payable and you can proceed to purchase. You'll be asked for a credit/debit card to pay with. You may already have a card registered with ManageMyMatch in which case you can simply use the one you have on file.