I am selling my stocks for the first time, what are some of the big impacts to understand when selling stocks?

Hey guys, It looks like I will be needing to sell a good portion of my investments for a financial endeavor in my family. I just want to know what are some things to take into consideration when selling stocks, mutual funds, index funds, etc.

I am 26 right now, I've been investing in stocks and funds for about 4 years now, more aggressively these 2 last years. But I've just been buying, buying and holding, I have not sold anything yet. But the time has arrived now, so what things do I need to handle? One thing I am thinking about is, that since this year is about to come to an end in a few days, would it be better to wait 2 more days? Because if I sell on the beginning of Jan, I'll have the whole of next year to pay any taxes right?