First, let's note a few things.
- Gunslingers in PF all have full BAB (Base Attack Bonus) progression. So assuming that you're level one, your BAB is currently at +1. This can be found on the table associated with Gunslingers.
- Your Dexterity score is 20, so your Dexterity modifier is a +5. There is a table in the Core Rulebook that lists modifiers with associated scores. Alternately, you can remember this equation to calculate it at will: [Modifer] = ([Score] - 10) / 2. Round this number down if you come up with a decimal. So for you, (20-10)/2 = 5. Nice and easy.
- Point Blank Shot applies a +1 bonus to attack and damage against targets within 30 feet.
- Precise Shot removes the -5 attack penalty for shooting at targets engage in melee combat. This is essentially a take it and forget it feat since all it does is let you ignore a penalty.
- Rapid Shot allows you to shoot an extra time while using a Full Round Action. All attacks made during this action suffer a -2 penalty.
- Musket Master will provide bonuses to attack and damage at later levels, but has no direct effect on either at level 1. It does however give you Rapid Reload (muskets). This is VERY good for you. A two handed firearm normally requires a Full Round Action to reload, but this feat lowers this to using only a standard action. Once you are level 3, this will be lowered again to a Move Action.
Anyway, all that information out of the way, now we can talk about calculating your Ranged Attack and Damage. These are both fairly simple. Ranged Attack is a summation of your BAB, Dexterity Modifier, and any other bonuses and penalties that apply (feats, range increment, buffs, etc.). For Ranged Damage, it's very easy. You roll a die based on your weapon and then add any bonuses from class features, feats or other such specific bonuses. You do not add anything to ranged damage by default. Certain weapon properties, feats, and other things will allow you to add to your damage, but you do not add anything to it by default. For your level 1 Gunslinger, we can do the math for a handful of situations now.
- When you make a regular ranged attack where the target is more than 30 feet away from you and less than 40 feet (the range increment of your musket), you add your BAB and Dexterity modifier for a total +6 bonus to hit. For damage, you'll roll the musket's die, a d12.
- In the case where your target is within 30 feet, Point Blank Shot kicks in. Then, your attack goes up to +7, and your damage goes up to 1d12+1.
- If attack a target that is 60 feet away, the range increment rule applies. The range increment is quite simple: every time you shoot farther than your range increment, apply a -2 penalty per X times your range increment by which you exceed it. So, since 60 exceeds your range increment 1 time, you apply the -2 penalty once to lower your attack bonus to a +4. For another example, let's say you're shooting at a target at 135 feet away. In this case, you exceed your range increment three times (once at 40 feet, again at 80 feet, and a final time at 120 feet, hence the name "increment") and apply the penalty three times. So your attack bonus would be at a +0 at this range. There is no penalty to damage however, so you still roll a d12 for it. As a Musket Master, you could also spend a move action to increase your range increment by 10 feet. In this example, this would increase your increment to 50 feet and you would then exceed your increment two times instead of three times (once at 50 and again at 100), so you would apply the penalty just two times. Then, your attack bonus would at a +2.
- Let's say you use a full round action to attack a target at 25 feet. There's no range increment penalty. Point Blank Shot kicks in and applies a +1 to attack and damage. Now, Rapid Shot applies if you want. Let's say you do want it to apply, so you can attack twice. You'll apply a -2 penalty to every attack you make this round. So with everything taken together, you'll roll to attack two times with a +5 attack bonus and roll 1d12+1 for damage when you hit.
That might seem overwhelming to understand, but that's only because I'm exhausting many of the possibilities that could apply with the options you have. If you think about this sort of thing ahead of time, you'll have less work to do later when combat starts :)
Now, I have to point out a problem that you haven't considered. You can't use Rapid Shot as a Gunslinger. You can't reload the firearm fast enough in order to utilize the feat at the moment. You'll need to get a special kind of ammo (paper cartridge) and then do one of two things: 1) Convince your GM to make a custom feat for you similar to a feat called Crossbow Mastery in order to be able to make multiple attacks per round or 2) Get a magic item that lets your reload faster somehow (which are not standard and again require the GM to custom make it for you). Otherwise, you'll never be able to reload a musket without needing to take some sort of action. This is the biggest limitation to using a musket: you cannot reload fast enough to make multiple attacks per round. However! It's not all doom and gloom. Go take the Deadly Aim feat instead of Rapid Shot, and you can increase your damage quite nicely.
Hope that answers your question!
