Josejanko,
You don't supply much info other than your machine is old, so here is some info for you to check thru.
Before doing anything you should have a back up, preferably 2, in the event something goes wrong.
You would have to start by upgrading to Snow Leopard, you would need to buy the disk for about $20 here http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
Check the the requirements to see if your machine meets them.
You would then need to run the 10.6.8 v1.1 combo update to get it to 10.6.8 Mac OS X 10.6.8 Update Combo v1.1
You can then access the APP STORE and possibly upgrade to to further OSX.
You may have applications that will not be compatible with any thing past Snow Leopard, you can check those here Upgrading to 10.7 and above, don't forget Rosetta!
http://roaringapps.com/apps
Office 2008 will not work with anything newer than Snow Leopard for an wxample
You may be able to upgrade to Lion or MT Lion or Yosemite, depending on your machine, from the APP STORE.
OS X Lion - Technical Specifications Lion
http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion Mountain Lion
http://www.apple.com/osx/how-to-upgrade/ Yosemite
Hope this helps you