I'm sorry to say this, but the fact that you don't know what is involved in the conversion process, suggest to me that at this point, you are not ready to go marine.
They require a mixture of deep pockets, attention to detail, great care, and a LOT of time. A marine tank is more a professional game whereas tropical is amateur in comparison. Once cycled, a tropical tank and its inhabitants can be very forgiving, a marine tank on the other-hand not so. Marine fish are also have notoriously strong personalities, which is not good for keeping a peaceful tank; they can change after months or even years of being fine, and before you know it, you have a second (expensive) setup just because your tang has suddenly taken a disliking to your blue angel etc...
You wipe out tropical stock (putting emotional attachments aside here), you are talking usually >£100 worth of stock. A single marine fish can cost more than £50. So if you have a decent setup with plenty of coral, urchins etc, you can be talking hundreds, or potentially thousands of pounds. Remember, practically everything is alive in a marine environment not just the fish, and this is key to sustaining a healthy eco-system.
Building up a marine eco-system is a very labour intensive and expensive task, and not for the amateur fishkeeper.
A good place to start would be the internet; I'd spend a LOT of time reading up on pretty much everything you can, no matter who you speak to or what you read, different opinions and findings will cause you no end of confusion; but you most certainly need to grasp the basics before even considering taking on a marine setup (and the basics would include what equipment/process would be required etc). I prefer the internet as opposed to books, because you are usually able to grasp a "majority opinion" on most subjects, and the people who post on them are generally fishkeepers, who have experience of what they're talking about; they're not there to sell you something or to bullshit you.
Also, take andys aquatics advice with a pinch of salt, they rarely give out good advice with tropical fish, nevermind marine. Their own tropical tanks are constantly under treatment, yet they are all running off a centralised sump, not very good for people buying fish and potentially introducing bad bacteria into their tanks?? On top of which, their staff are very geared to selling you something, so they will likely make out that it is very easy/simple to change over to marine, in the hope that you buy all your gear from them - shop around in any event, as things are far far cheaper online.
Remember, it needs to be done correctly, and you need to know what you're letting yourself in for prior to starting it; you could end up burdening yourself with a hobby that soon becomes a chore, and more importantly, (feel free to mock me lol) every fishkeeper must remember, that these fish and other inhabitants of the tank, are little people, with feelings and who feel pain etc, it's not fair to give them anything but the best possible conditions and setup to give them the best chance of a happy and content life.
If you do decide it however, good luck, and please post photo's of the development, would be fantastic to see.