Yes, you need an adapter to convert the 2.5" drive to 3.5". Here's OWC adapter. Included on that plage I linked to is a little video illustrating how to use it. If you buy some other adapter installation is basically the same.
As for how to make it your boot drive, there are a number of ways you can do that. The procedures make no difference whether is it a SSD or HDD.
The first way would be to do a full clean reinstall of your system on to the boot drive. Then if you need to, use Migration Assistant to move stuff from your "old" HDD boot drive to the new boot drive.
The second is simply clone your existing HDD to your SSD.
Once you created your new boot drive you can change your Startup Disk system preferences to default to boot from it.
No matter which method you use, always have backups before doing anything, or at least do not wipe out the original HDD until you are sure your system is fully working with the new SSD (wait a week), and then still create new backups of your new system.
Some other considerations. Persoanlly I recommend OWC for SSD. It's where I got my 120GB. Assume an Snow Leopard installation needs about 25GB to work with enough room to expand its vm space, etc. So depending on what you want to place on the SSD you probably be "safe" with anything from a 60GB to 120GB. YOu might consider placing your home directory on a HDD leaving the SSD for the OS and what you consider "key" files. That's how I have my system set up.
I agree with X423424X.
The size of the SSD you need can be calculated by subtracting your User directory from your total disk useage. Add another 50% for future growth, swap space, etc.
I did find it easier to do a fresh install and use Setup Assistant to migrate over the applications and settings.
Create a User Account on your SSD before changing the link to your HDD under Advanced Options.
Remember if you ever plan to merge your System and User Accounts onto the same drive (you may decide to get a larger SSD or go with RAID 0) using Setup or Migration Assistant, a valid System Folder is required on the drive with the User Accounts.