Lessons of Hemmingstedt
- is Dithmarschen.
Expanding in the HRE
You're too weak to really take territories from Denmark first, so your best option is to expand in the HRE, to the north-west of it. Since Dithmarschen is a republic (with strong trade bonuses), aiming to take the province of Holland is a good option to start making a decent income from trade. The alliances you make should depend on which are the rivals of the countries you want to take territories from. You should also be extra-careful that Austria, as the emperor, could attack you if they claim an illegitimate territory. So improving your relationships could avoid the risk of an early war against the Emperor.
Anyway, it's best to develop slowly and to take the best territories inside the empire, depending on their development level and the trade power they would give you.
The fall of Burgundy
The fall of Burgundy generally happens early in the game, but you may need to help it. Allying France or Austria are the best options to do so, because they're probably already rivals of Burgundy. Your aim is to force Burgundy release Holland from being a subject, or to take the province of Holland from Burgundy. Once you've got Holland, your income should increase significantly from trade. To do so you must redirect all the trade power to the node of English Channel. You can take Trade ideas as the first idea group to increase significantly your income from Trade.
Once you've grown strong enough, depending on the situation, you may be strong enough to attack Denmark. If Sweden wants to go for independence, you can support it to decrease the power of Denmark. Muscovy might support Swedish independence as well.
When Protestantism appears, you'll have to choose which religion you want. According to the position of your country, it can be difficult for you not to go Protestant since centers of reformation appear close to your country.
When Sweden is independent, you can start to make claims on Denmark to take its territories. Since you'll first attack Denmark from the west, you can take its provinces on the continent before taking the Islands. By doing so, you may reduce the size of its Navy, which could help you in future wars to take the island of Sjaelland. Sweden is the safest choice to take as an ally since it will always be the rival of Denmark. Depending on the other rivals of Denmark, you should choose carefully your allies before attacking.
It might be easier to attack an ally of Denmark and to force Denmark in a separate peace if Denmark as too strong allies. Denmark shouldn't resist too long.