If you've got some fat around the base of your penis, you might want it gone. Perhaps to use your full length, or just so it looks better. Whatever the reason, you can get rid of that fat.
The build-up of fat at the base of the penis is called the pubic fat pad. It's one of the first spots on the body to add fat, and when losing weight it's one of the last to lose it. Pubic fat also occurs in women, and it can serve a very important purpose: a bit of padding for for hard and fast sex. If the fat wasn't there, you'd be smashing just a skin-coated pubic bone against another skin-coated pubic bone, and I don't know if you've ever whacked your elbow on something hard, but that can hurt like the dickens.
That's not to say that there's not reason to reduce the pubic fat pad to a more reasonable depth. While body fat is highly compressible and with a small amount of force you can push down the fat around your penis, it's not infinitely compressible. So with less fat around the base, more of that base can be used (and more penis can be measured). And if we're being vain, it looks better and can make the penis appear bigger by exposing more of the shaft to broad daylight.
How to get rid of pubic fat
The bad news here is that there's no way to target fat pad loss with exercise or diet. You could opt for liposuction to remove some of the fat cells in the area, but for enduring and more natural results you'll need to make lifestyle changes.
The hard part of wanting to lose weight in a specific area is how the human body handles fat burning: it burns a little fat from everywhere, all at once. And even if you could, there aren't muscles that you can really bulk up in the pubic region anyway.
So what to do? Eat healthier, eat less, and exercise more. It's simple, but difficult, because it requires long-term dedication and determination in the form of a permanent lifestyle change. While there are plenty of different diets out there, there's no one miracle diet that we can recommend. I'm not a nutritionist, but I am smart enough to know that everybody's bodies are unique and will handle different diets in different manners, and you need to pick something that works for you. But in general: reduce your intake of red meats, sugars, and heavy starches, and eat more vegetables and lean proteins.
The human body has a baseline caloric need every day. Even if you just laid in bed all day you would burn well over a thousand calories by just existing. The more you move, the more calories you need to burn to accomplish that activity. And if you're burning more calories than you're taking in, your body will turn to its fat reserves for extra energy. That's the whole point of body fat — when your stomach is empty there's still an energy reserve.
Exercise is a great way to burn more calories than you take in, and cardio exercises are one of the best ways to do that. Any endurance exercise, from running to biking to rowing will kick continually burn calories the entire time, and working the heart will do wonders for your boners.
The name of the game is "calories in, calories out".
How much weight do I need to lose?
The question you might be asking yourself is how much weight do I have to lose to expose my entire penis? And despite what you might've read online — every 40 pounds = 1 inch exposed — there's no single answer to this. Every guy carries weight differently on their body, and every body burns fat in different areas at different rates and at different times.
The real answer is that you should focus on your overall health and fitness, and let any pubic fat reductions happen be a bonus. Getting slimmer and fitter will have great benefits overall for your health. You'll sleep better, you'll be less tired through the day, you'll look better, you'll probably just feel better about yourself, you'll be less exhausted by physical activity, and so on. And yeah, the further you go, the less fat there will be around the base of your penis.
So go to the gym, hit the trail, hop on your bike, peruse the produce section at the grocery store — make the lifestyle changes you need for a healthier you. And maybe you'll see more of your dick at the end of the day too.