My 2003 Toyota Tundra just started having a lifter knocking how
do I fix it
Noisy lifters can be caused by a dirty gunked up engine not allowing lifters to fully expand with oil pressure, or can be caused by worn parts (like cam lobes), or timing chain issues (like slack). Oiling issues you can try an engine cleaner like Seafoam, add some to the crankcase, drive 100 miles, then change oil. May or may not help, depending on how severe the issue is. You can also try other engine additives but again their degree of success depends on how severe the underlying issue is. If it's just a collapsed lifter you can replace them, it's a basic operation any mechanic can handle. If it's something more severe like a timing chain or worn cam then you can only fix it by opening up motor and replacing parts. Worn motors with a lot of miles on them do need a little extra TLC in the way of oil additives, so look into those.
I would try an engine flush such as LiquiMoly. You have nothing to lose.