How do I fix my translucent/transparent teeth?

Based on your photo, your teeth are translucent but also need to be professionally cleaned. Depending on various factors, bonding material may be able to be added from the inside to help with the translucency. MI Paste or a strong Fluoride toothpaste can help with the sensitivity, but you should see a dentist to evaluate everything. 

You need to go to your dentist and have him evaluate why your 2 front teeth are so translucent.
Do you drink or have you eaten food that has acid like coke or lemon that might cause this erosion.
You also need to have your bite checked because this may be causing the problem.
If it is your bite, then you may need more space on inside of  your tooth to repair. This would be accomplished by orthodontics or invisalign or possible adjusting your occlusion.

You then can restore your teeth with porcelain veneers or bonding.

Based on the photos, you appear to be a good candidate for composite or veneers, but first I would recommend a cleaning. Seek an experienced cosmetic dentist who has great photos and reviews.