There are generally 3 ways of someone being able to access your web space and change files etc.
Most obvious one is if you are careless in leaving your FTP password and username laying around for people to find, saved within your application , how many people have the remember password box checked! or even worse actually giving out your password to someone else. Anyone will be able to access your web space if they have these. So like all passwords guard them carefully, and take steps to ensure they cannot be guessed easy such as not using your name family or pets name, dates of birth, or even worse using the same word for login name AND password!
Another way your web space will come under attack is if someone attempts to hack into it via the server it is stored on. The security of these is down to the web hosting company, Generally all companies are very hot on security as anyone gaining access can create havoc not just to your site, but to the hundreds of other sites they have, including thier own. There is no extra charge for secuirty, everyone should be protected the same. If your site is a personal site, then you will stand more chance of winning lottery than being targeted at random by a hacker as most tend to go for large companies and government departments .
There are other minor ways in which someone can screw your site up, most common of these is if you have a message board/forum where HTML is ENABLED. People can write code to crash the board and other annoying things.
If you have anonymous FTP access allowing uploading (a lot of sites have a domain name such as ftp’dot; domainnam’dot;com this is Microsoft’s site: click here , which allows people to download and sometimes upload files direct instead of through site links.) someone could upload a potential harmful file, but I doubt you will be using public ftp on your site, if you do just set permissions to deny any uploading. If you haven’t a clue what this all means then its something not to worry about because you wont be using it!
I wouldn’t even think twice about your site being attacked there is no reason why yours should, unless someone has a personal vendetta against you and is hell bent on causing you problems. As long as you just keep your details safe then you should never encounter any security problems.