First of all: Get loose leaf black tea, trying out several varieties to find a baseline one you like. Black tea is the simplest to brew of all the caffeinated tea variants. You'll also want to experiment with water temperature, steeping time (not that long, else you'll get the bitter flavor bits that no one like), and how you go about steeping (i 've found that disposable tea bags that I can fill with loose tea is best for me). I've fine tuned this process for myself to the point that I can make super strong tea that also naturally tastes sweet!
The mistake that many people make with tea vs coffee is that they equate the jitteriness with the caffeine working. In tea, there is also theanine, which moderates the jitters that occur due to caffeine via being a very mild muscle relaxant, and I am lead to believe that theanine has been found to have wholely positive cognitive effects generally.
That being said, coffee or tea are only helpful if you have a caffeine dependency, or you are amidst being tired/groggy to an extent that interferes with work. If your issues with doing work are not those, drink the tea anyways because its tasty, healthy and easier on the stomach/body than equivalent volumes of coffee (also tasty) or sugary(real or fake) energy drinks. and/or go talk with a doctor to figure out why you can't do work.
