jailbreaking will never ruin or break your iphone so no need to worry about that. The only risky part is not knowing what the packages you download actually do, this may cause conflicts and lead to instability on your phone i.e it may start in safe mode a few times.
with cydia (the jailbroken version of the app store) it asks you what type of user you are when you 1st load it up, here if you click 'user graphical only' and not developer, then most of the programs that could cause problems will be filtered out.
I would advise you try the jailbreak as it is an easy process and you get a lot out of it. The worst thing that will ever happen is you will need to restore your phone via itunes as mentioned before, although this can be a pain if you have a lot of jailbroken apps but there are ways around having to reinstall all of them too, i would recommend a cydia app called PKGbackup, always worrk reaslly nice for me.
So to clear things up
1) sync with itunes to backup
2) jailbreak
3) install and use cydia as a 'user' not developer to download what you want
4) back up jailbroken apps with PKGbackup and sync regularly with itunes
Enjoy ur new phone!!
if you ever run into troubles
1) sync with itunes (if you can)
2) restore via itunes
3) restore from backup on itunes (it will automatically ask you to do this) this will reinstall all ur app store apps, photos, music, text etc
4) re jailbreak
5) reinstall cydia and download PKGbackup again
6) run PKGbackup to restore jailbroken apps
its not tough especially after ur first try, u learn as you go along like i did, so i would recommend giving it a go!!
Hope this helps