Make exercise fun. Tiredness can come from many causes: poor nutrition, sleep apnea, various medical disorders such as anemia, diabetes, or thyroid disorders, psychological factors such as stress and/or depression, etc. See a doctor if you're "always" tired. To add motivation for exercise, try changing up your routine, exercising with a partner or trainer, or make it "fun": bicycle, hike, etc. Something you like!
Could be many things. Before doing anything more a good physical exam comes first. There are multiple medical reasons for being tired all the time. Endocrine, cardiac, blood disorders, neurological issues to name some. If no major medical problems are found then the next logical area to look at is psychiatric problems, mainly depression. Lack of energy, drive and loss of interest in any physical activities are common.