How do financial arrangements and other legality s work with Marriage ?

Marriage for you two needs to wait another 5 or 6 years, if not more. It's way premature to think about it now. By the time you're ready (through school, onto your career paths, financially stable, mature, etc) you'll have a LOT better idea about how things work and what you want out of a marriage.

Financial arrangements work however you two decide for them to work. Different couples do different things. Some have 1 joint account that all money is put into, and all bills, expenses, and purchases come out of. Some couples have a joint account that they each contribute a portion of their earnings to, for the payment of bills and whatever they decide, while also each maintaining a personal account with money to spend as they need/desire personally. Some couples maintain totally separate accounts and decide who will pay what bill(s) in full or they split things down the middle and each pay half. It's up to you and your spouse to decide what will work for you.

Taxes work a lot like they do for you now when it comes to income taxes. You just decide to file separate or jointly and make sure you look up the proper tax amount in the tax table for your situation (or pay someone to do your taxes for you). Insurance depends on what kind (auto, home, medical). Rates are usually lower for auto insurance when you are married. As for medical, depends on if you get private or through your employer.

There's not a lot of unusual or extra stuff you have to worry about regarding being married versus living single or living together unmarried (from the taxes, insurance, legal standpoint).

Legalities

Separately, after you graduate from high school, get jobs, register for colleges and move out of your parent's homes and into separate places. So that, separately, you'll learn how to earn money, how to handle money, how to budget, how to pay your rents and afford food and socialize and have roommates and then, 3-4 years from now... you'll have a chance to see if you are compatible as adults and would be capable of taking care of your business as a mutual endeavour. Don't worry about getting married until you are functioning adults.

It's good that you want to get all your ducks in a row.

I suggest waiting till you're 25 and REALLY have your stuff together.

You will be a different person (no teenager really believes that, but it's true) and you have a MUCH better chance at a successful marriage at that age.