In answer to your question:
1 . My server has a distinct IP, can I use it although I have other sites
there but they don't need ssl?
There is really no mucking about when it comes to SSL, your site will need its own IP address for a single domain SSL. There are various hacks and bodges I've seen people do over the years, they all end in tears.
2 . If I do need to buy dedicated IP. What does it involve?
If you're self hosting your server in an office/home at the end of a DSL or cable service then you need to ask your provider for a range of static IP addresses. If your server is rented from a hoster (RackSpace, Orcs etc) or it's your own hardware in a data-centre then you'd need to ask the hoster or IP transit provider for more IP addresses.
As to cost, this can vary from one-off payments of around $50 for a /29 (6 usuable addresses) to annual renting of an IP address of perhaps $10 per IP/per year. It will vary enormously from provider to provider.
One thing to note is that if you change DSL/Cable/Hosting/Transit provider - you can't take the IP addresses allocated with you. They are part of their larger allocation from a regional internet registry and "owned" by them.