Don't know your phone options, but if you go to Menu > Settings > Display > Font are there Font Style and/or Font Size settings which you can alter to resolve your problem?
Have you installed any new apps and noticed that the font changed after installation? if so, uninstall the app to see if the problem is resolved.
If the above do not work or are not available try resetting your phone. Backup the phone using the phone's backup feature first , then perform a factory restore using the phone's restore to factory feature.
Be aware that this will reset your phone to a factory default condition. It will erase all of your downloaded apps and user data. That is why it is important to do a backup first.
Once a reset has been completed, check what the font is like. If it is back to normal, restore your phone (with the backup you performed earlier) using the phone's restore feature.
If the font problem has returned then it is most probably a downloaded app that is causing the problem. The hard part is finding out which one. You will have to uninstall each one, one at a time, until the problem disappears to know which one it is.
If the problem didn't return after you restored the phone then perhaps it was just a software 'glltch' which was resolved by the reset.