It is so much more healthier to prepare and cook your own vegetables at home. We cannot be sure how clean the vegetables outside – are the vegetables washed frequently?
As well as food additives like MSG, too much salt/sugar/oil are used to make the food taste better. The oil used (often recycled oil from frying meat is being used to cook the vegetables).
One of the first things I learn when cooking dishes is how to stir fry vegetables because it is easy to do and healthy. Being a person who naturally do not like to cook, I find it easier if I were to prepare my vegetables and cut my garlic into cloves before hand so it only take less than a minute to take out the ingredients and less than 10 minutes to have the finished dish on the table.
Here are my experience sharing when it comes to buying and storing vegetables:

1. Buy enough- don’t buy more than you can eat which ended up as food wastage and that forces you to waste time to declutter your fridge.
In the past, when I did not know better, I tend to buy much more vegetables than I would care to cook. Look at the picture below- I bought some siew bak choy from a hypermarket just because it was cheap. But the packet of bak choy ended up rotting in my fridge:

Knowing that a small bunch of lettuce is enough to last me a day (no point of cooking a lot of vegetables as my other family members do not like to eat vegetables), I tend to buy about one or two lettuce and a bit of other vege each time when I visit the market instead of buying a big bag which I would end up throwing. And I really do not like to waste food.
2. Prepare your vegetables before hand (so that you do not feel too lazy to cook them)
This simple action made a difference and I cook vegetable dishes because the prepared vegetables make it so easy to just take out and cook.
Usually, I would soak vegetables that I plan to eat a few hours later or the next day. For example in the morning I would soak some vegetables that I wish to eat for lunch. I soak them 3 times- the first time to rinse through, then the second time I soak longer and third time I wash again.
If I am not planning to cook the vegetables within the same day I would sieve them to drain the water and then line a container with kitchen towel before putting the vegetables inside like below:

I find that it makes the vegetables last a little longer. But not all vegetables can be plucked off and stored like above. For example, lettuce does not do well being stored in the fridge for more than a day so you would need to wash them a few hours before cooking.
I stir fry most of my vegetables the Chinese style hence I use garlic.
I would peel off the garlic and store the garlic cloves in a small container, then put in the fridge. I only cut up the garlic into small pieces when I want to cook the vegetables. Hence each time I want to cook the vegetables, I just take out from the fridge, and separate between the stalk and the leaf (as you need to cook the stalk first). Then I put in salt, sugar and oyster sauce on the vegetables and prepare little bit of water nearly. Then cut out a garlic glove or two and proceed to stir fry the vegetables.
Conclusion:
Because now I buy just enough instead of in huge excess, it frees up my fridge space and I always know how much of fresh produce that I have inside my fridge. It also help to reduce wastage and save money – I have learnt not to buy a lot just because it is cheap because eventually it made me feel a little sad to throw them away.
If you have just started to cook at home, don’t be stressed when you initially could not estimate well. With trial and error, you would get to the ‘groove’ of being able to estimate better.