The site
To have a micronation you need to be able to claim a section of land. For instance, you could create your own living space:
You can also claim territory on the seafloor or in outer space. The main advantage of these is that you can claim these for purposes of research.
Flag & Coat of Arms
Independence
See Independence
Political and legal decisions
Constitutional state
Next, you have to write a constitution. If you don't know where to start, look at the constitution of the United States for knowledge of what to include. You can also include a provision for amending the constitution if you wish. You can stop at this point, but we recommend going on to step four.
Establishing sovereignty
Now, you should establish your sovereignty and make your name and that of your micronation known to all who wish to know it. A website is a useful tool at this point; we recommend using a site like Freewebs to create the homepage and, if preferred, other pages about your brand new nation!
National Defense
Defending your own territory against outside forces (pirates, for example) is a tough problem to overcome if you do not have the means. If you have made treaties with other, more powerful nations that require them to aid you if you so ask of them, this may be of great benefit in terms of defense. However, in the case of a small man-made island, the outcome of an attack against your island depends greatly on whether the other nation's forces arrive to your aid on time or not.
You may happen to have the resources to build your own defense force, too. It would be a good idea, firstly, to set up signs around your island to warn outsiders of your existence and your right to defend yourself. Creating a defense force requires, most basically, firepower. Missile launchers, cannons and the like are useful for relatively long-range attacks across the water, however, this may require a strongly built island with enough room to store ammunition and hold up to recoil, as well as some way to repair or evacuate the island in the case of an ammunition explosion.
However, even if you have the firepower, the next important thing to have are those who will defend. National defense is not usually a one-man job, and it would be preferable to have more people working in the defense force. This is where having a small (or even better, large) population would come in handy.
If you have claimed a sufficient area of territorial waters for your island, it may then be practical to set up mines around your island. However, this may hamper your own transportation on sea if you plan to do so in the future. Having air-transportation (i.e. helicopters, etc.) would not be a problem, though.
Planting mines *may* be against international law; you must declare that you have mined an area, but how specific you are in declaring it is completely up to you.