If you or your master at arms have reached at least level 20 and have some spare ability points, you can get the "Landing Party" ability. That allows you to attack unfortified colonies such as Pirate Settlement on Nevis. This is a lot easier than taking a fort, and should also get you plenty of points towards your next promotion.
Whether you're attacking forts or unfortified colonies, you don't need to capture the town to earn lots of points. Ransom it instead. You'll still get lots of points, you'll also get a lot of money, you can loot the store, and then later on you can do it again.
As for a 1st rate, you don't need to buy it. If you're lucky, you can meet one in an enemy fleet. You'll want good armour and good swords for you, your officers, and your boarding crew - make sure your weapons locker is well stocked. You and your officers had better have good melee skill and abilities too because you'll be facing a lot of opposition!
Alternatively, if you're playing the "Ardent" storyline, play the story! Everything I just said about being well equipped and skilled applies because you're going to have to take on a 1st rate as part of the story. I've personally taken it using a fast war galleon. And I know at least one other player took it using a frigate.