There can be many reasons why you might feel tired. Going to your GP is a good place to start to understand why. They can give you a general check over and maybe do a blood test too for vitamin and mineral deficiencies.
Some good advice on here like getting a good diet but I came to the conclusion many years ago that I was perhaps pre-disposed to just feel tired most of the time and assumed many others were the same
I have to say however that while I don't have the same energy levels and general Zest / Zip for life that I used to have when I was younger I also now don't feel excessively tired all the time
What changed for me was giving up a job that was quite stressful to the point that I sometimes felt dead on my feet / totally wiped out for months on end, winter and lack of sun was also always a bad time for me and I used to get SAD until I started going to the gym after which point I never had it again
A few comments about going to the doctor but personally i'd feel like a right time wasting twat rocking up and saying I felt tired, used to have a couple of friends as a kid whose mum would take them to the doctor if they had a cold, the doctor must have rolled his eyes but perhaps they have plenty of spare time these days