Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
Whilst Apple have withdrawn Snow Leopard from their stores, you can still get it from Apple by calling 1-800-MY-APPLE (if you are in the USA) and they will supply the SL DVD for $20 for a single user, or $30 for a family pack that covers up to 5 Macs. You can also purchase the code to use to download Lion from the same number (Lion requires an Intel-based Mac with a Core 2 Duo, i3, i5, i7 or Xeon processor and 2GB of RAM, running the latest version of Snow Leopard), or you can purchase Mountain Lion from the App Store - if you can run that:
If you are outside the US call your national Apple Helpline:
If you're in the UK, use this number: 0871 508 4400
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.
To use iCloud you have to upgrade all the way to Mountain Lion:
Is your Mac PPC or Intel? If Intel, you may be able to run Snow Leopard, which many of us are still using. No need to go beyond that, especially since you will lose support for older PPC apps if you do.
The basic requirements for Snow Leopard are as follows:
An Apple computer with an Intel Processor
1GB of Memory (RAM)
5GB of hard drive space (Storage) DVD drive for installation
I would double the recommendation for RAM. Much more even is advisabe.
To find out what processor your computer has click on >About this Mac
There is no shortage of hype from product vendors happy to take money from those who want to part with it.
I am never going to buy a new Mac or another Mac OS.
I am very happy with my secondhand 733 MHz G4 towers running Leopard (OS X 10.5.8). They have 1.5 GB of RAM and upgraded double layer DVD writers (£20). I use SeaMonkey (latest Firefox and Thunderbird combined) for web and email. My inexpensive TalkTalk phone/broadband gives me 15 Gb/s download speed and, despite not having a static IP, is stable enough to run a permanent G3 web/DNS server.
Before I retired I was a webmaster for over 10 years at a university and had access the the latest computers and networks including my relatively recent Xserve and desktop PC with Ubuntu.
I plan to move my server to a Celsius R630 with Ubuntu which I bought a while ago for £100 but I will probably continue to use my G4s for many years to come.