You will need to create an amendment that you would like to see, or choose a part of the Constitution you would like to see amended. Two that have been discussed in recent years, albeit mostly by the media, have been an amendment to allow those not born in the United States to become president, and an amendment that would allow marriage only between men and women. The first was discussed largely in conjunction with the idea that Arnold Schwarzenegger might run for president, and the second in conjunction with the ongoing culture wars over the rights of gays and lesbians in what is supposed to be a secular democracy.
As you decide what kind of amendment you wish to propose, you need to be aware that it is extremely difficult to get even a proposal through Congress, and that it is even more difficult to get the requisite number of states for approval. There needs to be a compelling reason that is really above political interests because the country is presently so solidified into it "red" and "blue" positions. The founding fathers, in their great wisdom, realized that the Constitution should not be able to be easily changed.
A formal method of changing the Constitution is by amendment, and there are a few ways you can do this. The most common way (26 of the 27 amendments) is for 2/3 of both houses of Congress to pass the amendment, and then 3/4 of the state legislatures ratify it. The amendment can also come from the states in conventions to the national level, but the 18th amendment prohibiting alcohol is the only one that was approved in that manner.
Informally we can change the Constitution through court rulings and interpretations, whereby the court identifies or validates an implied power not directly written into the Constitution.
If I was able to write a 28th amendment at this point, I would write one for campaign finance reform that outlawed all private donations to political campaigns and required public funding instead. In light of the recent Supreme Court ruling and the increasing role of money in campaigns, some sort of reform is necessary to make government more accountable to individuals and not merely the wealthy or businesses.