First of all, know that you are not required to give up anything specific for Lent (or give up anything at all, really). All you are required to "give up" during Lent is meat on Fridays and Ash Wednesday, and food (in the form of fasting) on Ash Wednesday and Good Friday. Fasting means only having one meal during the day, though it does not preclude taking other food if necessary so long as it does not equal another meal. Catholics 14 and older are bound to abstain from meat, while Catholics ages 18-59 are bound by the fasting law. All things considered, that's not much.
So Why "Give Up" Something?
If all that is required is what is mentioned above, why do Catholics typically give up other things during Lent? It's because Lent overall is a season of fasting, prayer, and charity. Fasting should be part of our Lenten experience. That's why we are required to fast on Ash Wednesday and Good Friday, marking the beginning and ending of Lent. But it's good to fast in other ways all through Lent. The US Bishops recommend fasting on all weekdays of Lent. This won't be possible for everyone, though. So most of us will choose to fast in a limited way by voluntarily giving up something, usually food-related.
So you may choose to fast from desserts, from snacking between meals, from meat, from coffee, from alcohol, or some other type of food.
It's Not a Diet
Keep in mind our fasting is supposed to be for our spiritual benefit, not necessarily our health. If you want to give up carbs so that you can lose a few pounds by the summer, that's a diet, not fasting. Dieting for your health may be a praiseworthy endeavor, but that's not the point of the Lenten fast. Our Lenten fast is about both doing penance and also disciplining ourselves to learn to resist bodily pleasures. By denying ourselves something good that we desire (like chocolate or coffee), we learn to deny ourselves more illicit pleasures when the temptation to sin arises.
With that in mind, the thing you choose to give up should be something good. Otherwise it is not a sacrifice. It should also be something that you feel attached to in some way. It should be something you will miss. If you only drink a couple of beers on the weekend, then giving up alcohol for Lent won't be much of a sacrifice for you. You may not even notice it. But if you habitually eat dessert after each meal, giving up dessert will have a great impact on your daily life.
Try to choose something that you will feel the absence of each day. You want it to be difficult to give up -- but not impossible. Don't set yourself up for failure. You want your Lenten sacrifice to be hard, but not too hard.
Remember, too, that your Lenten fast is self-imposed. Apart from the requirements mentioned above, what you give up is up to you. That means you can make changes as you go, if you feel they are necessary. If you start out Lent by giving up caffeine, you may find two weeks in that it's much easier than you think. You don't miss it at all. It doesn't really feel like a sacrifice. Perhaps, then, you should consider giving up something else.
Alternately, you may find that without caffeine, you are especially grouchy. You feel miserable, and are making others around you miserable. It starts to negatively affect your friendships, or makes it very hard for you to study. This may also be a reason for giving up something else. Your Lenten sacrifice should be a sacrifice for you not for those around you.
Think Outside the BoxWe typically think of giving up something food related, because of the connection to fasting. But you are free to do penance in other ways. One year my pre-teen daughter gave up her bed, sleeping on the floor of her room all of Lent. Some people will give up Netflix or social media. I had a student once who gave up eating with utensils.
Some will suggest giving up your time by devoting extra time during the day to prayer, spiritual reading, or doing charitable acts. These are all good things, and go right along with the Lenten practices of prayer and works of charity. So I'm not saying don't do them. Definitely do them. But, in my opinion, they don't really address the spirit of fasting. Fasting calls us to do without. It reminds us that the material things of this world, as good as they are, are not the greatest good. By voluntarily denying ourselves the happiness we get from food, drink, or other material things, we learn to turn to God as our primary source of happiness, and so grow one step closer to that eternal happiness we are called to enjoy forever in heaven.