There are three main areas that the Church traditionally gives us for choosing Lenten practices: (1) Prayer, (2) Fasting, (3) Almsgiving.
I recommend picking one thing from each area. When you choose something, it should be small, concrete, and doable.
It should be concrete. When I make vague Lenten promises like, “I’m going to pray more”, then nothing ends up happening because I haven’t ultimately committed to anything. Rather, say something like, “I’m going to spend 15 minutes reading and reflecting on half a chapter from the Gospel of John each day.”
It should be small and doable. Don’t give up coffee if giving up coffee is going to make you completely unbearable to those around you. Your penance shouldn’t be a penance for others.
Don’t make grandiose plans that are not sustainable. One Lent I started with 10 push ups and 10 sit ups a day and was going to add 10 more each day during Lent. By the end, I would have been doing more than 400 of each per day. By the second week, it became too much and I gave up.
So you might give up snacking after dinner, spend 10 minutes a day reading Scripture, and give an extra $5 a week at Mass. That is just one example.