I'm going on exchange to Japan during my sophomore year with AFS.
First off, I recommend that you rethink your country choice. Yes, some English speaking folks do go on exchange to other English speaking countries, but I personally believe that if you're even bothering to go on exchange, you should go somewhere a bit more different, somewhere where you'll constantly challenge yourself with language and overall cultural differences . In my opinion, that's where the beauty of exchange lies. Of course, it's ultimately up to you, just my two cents.
Now, moving on to less opinion based, things...
There's many exchange programs out there, but you're better off with one of the big three: AFS, YFU, and Rotary (in no particular order).
AFS and YFU tend to be a little on the expensive side, but are worth it, especially if you're set on a certain country (AFS no longer offers England as an exchange destination, by the way). You can expect little application difficulties with either one, as long as you have decent grades (2.8. GPA minimum).
Rotary's different, but just as safe and reliable. It's much cheaper (actual prices depend on your local Rotary club, but they're very affordable in comparison to AFS and YFU) and is more group based than AFS and YFU which tend to be individual based. By this I mean that AFS and YFU separate their students from each other. AFS and YFU students are handled as different individuals, each AFS/YFU counselor minds his/her own business. On the other hand, Rotary is more of a "you look after me, I'll look after you" environment and their students are addressed as a group. In the end, AFS and YFU are not better than Rotary, and Rotary is not better than AFS or YFU. The three are great programs, and it's all up to personal preference. Some might prefer Rotary's family approach, while others might like AFS or YFU's approach better.
So, moving on to what you have to do in order to be an exchange student...
Assuming that your grades are at the level they should be and that you're looking into a year program, all you truly have to do is choose your organization. Sounds easy enough, right? Not really. This is because your organization has to choose YOU. This means a pre-application fee to pay, plus a pre-application to fill out. If all went well with your pre-app, you will then have a full application to fill out (and these tend to be long). You'll have another fee to pay, and if they accepted your full application, you should expect one or two interviews. The intensity of the interview depends on your organization. AFS and YFU interviews are usually nothing compared to Rotary ones (although, Rotary interviews have always sounded quite fun, in my opinion. They tend to ask tons of scenario questions, e.g., "what if...this? what if... that?"). That's all because Rotary tends to be more competitive. Anywhoo, after that, if you're accepted, your organization will help you with your VISA and other important things like that, and...then the waiting process will begin, during which you anxiously wait for your host family information and the day of your departure. Hooray!
If you have any more questions, feel free to contact me.