You're right about the lemon juice and the milk but wrong about the sugar. Lemon juice is weakly acidic (hence its sour taste) and milk is slightly basic (or alkaline, the terms are used interchangeably). A sugar solution is completely neutral since sugar can't take hydrogen ions out of the water or donate them in.
If you don't have background in chemistry, then let's not talk about pH - I thought pH was confusing when I learned it because "lowering the pH" makes a solution more acidic, and "raising the pH" makes it more basic! (Ironically, the pH scale was originally devised to make the concepts of acidity and alkalinity more accessible to the general public)
Your observations about tea probably have nothing to do with acidity or alkalinity since the foods you've described are not significantly acidic or alkaline (except lemon juice). Tea is slightly bitter because of tannins in the leaves. You've evolved to think of things that taste bitter as unappetizing because they often do not have nutritional value and may even be poisonous.
Sugar (and sweet things in general), on the other hand, is very tasty because you've evolved to know that consuming sugars (carbohydrates) will give you energy. Milk also contains sugars, as well as lipids (fats) which give it its creamy taste, which you've also evolved to find appealing because they give you energy - more than simple sugars.
The creamy taste of milk or sweet taste of sugar are usually enough to cover up the bitter taste of tannins (if the tea is brewed right, that is, but that's an issue for another discussion) and make your tea tasty.
