Absolutely, it will help. As far as I remember, the HTC Sensation is very similar to the HD2, hardware wise that is. That's a great thing because the HD2 is a legendary device, and it runs the latest version of Android even though it's 3 years old now.
Anyway, to answer your question, a custom rom will speed your system up depending on what you use. If you use an AOSP (Android Open Source Project, or simply vanilla android) based rom it should become faster because HTC Sense is very, very heavy on the device's dated hardware and the cache build up over time could have been responsible for the slow down.
If you aren't looking to customize it too much, Cyanogenmod would be the way to go. It is probably the most popular AOSP custom rom distribution, so you just can't go wrong with that. If you'd like to have complete control over your device, for example statusbar transparency, you should get Paranoid Android. However, I would not recommend it just yet because you are a beginner.
It goes without saying but you'll have to of course, back up everything. The internal sd card will likely get formatted so that should take first priority. Also, your apps and contacts should be synced to your Google account, so they'll automatically start downloading once you install the custom rom and connect it to the internet. That shouldn't be a problem. You'll also need to get a custom recovery in order to flash cyanogenmod.
Oh also, rooting is basically getting read/write access to the file system. If you're planning to install mods/edit stuff etc., you'll need to be rooted.
Here are a few resources you can refer to. They'll help you get started.
Beginner's guide to getting started (rooting, flashing custom recoveries, roms etc.):
http://forum.xda-developers.com/showthread.php?t=696189
Cyanogenmod 10.1 for the HTC Desire:
http://forum.xda-developers.com/showthread.php?t=2070704
Read through the first few posts by the OP in both threads to understand what they're about exactly. These are basic guidelines that should help you, but if you need further assistance, feel free to ask!
EDIT: Just realized I replied to a 4 month month old thread, sigh. Hope all of that helps someone.