What was the Abraham Lincoln's first main idea?
This is a rather vague question, but I will try to guess at what you mean.
I think that you might be asking what his main goal was in the things that he did as president. If so, I would say that his main goal was not to end slavery, as many people think, but rather to keep the Union together. He wanted to prevent the US from splitting up.
Lincoln himself said that his main goal was to keep the country unified. He once said that if he could do it by freeing the slaves, he would, but that he would also do it by not freeing the slaves, if that is what it took.
Lincoln's single most important goal was to keep the US a unified country.
In case this question means something else, such as, what was his first cause or idea in his political life, that would be to stop the expansion of slavery into the newly acquired territories in the west. Lincoln was first elected to Congress in 1846, as a member of the Free Soil Party - the Republican Party would evolve out of this, but at the time did not exist - Lincoln opposed getting into the Mexican-American War, and was against expanding legal slavery west of the land it already existed in.