If you are still running Tiger it must be an old computer.
Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
OS 10.6 Snow Leopard is once again available from the Apple Store:
and in the UK:
but nobody knows for how long it will be available.
When you're offered installation options, ensure you choose Rosetta to be installed or your PPC apps won't work.
The installer will replace the Tiger system files with SL ones and should not touch any of your data or settings. Note that the restart will take considerably longer than usual as files are moved from a temp folder to their correct places in the system.
Caveat; I did 8 of these at the beginning of the year, but two wouldn't install until I erased the Mac HD volume. No idea why the difference, but it does make that cloned backup important.
Be warned that printer drivers, and scanner drivers in particular will almost certainly need updating for SL. That could be a problem if the printer or scanner is more than a couple of years old.
Many third party applications (and some Apple ones) may need updates for SL (Adobe particularly), so check with the vendors for compatability before you upgrade.
There is no Classic in SL and no support whatever for OS9 and earlier applications.