Recently, collecting movie posters has become a fashionable hobby. Movie posters were made by artists and are artworks. Collecting movie posters is the same as collecting any other collectible.
You first need to decide your budget. The sweet spot for a movie poster is usually in the range of $2000. Take care that you do not spend more than this, unless it is very highly-valued. You should know the different factors that are used for classifying posters such as their size, genre, type of material etc. Once you know this, you can begin searching for posters. The internet is a very good place to begin searching. There are a large number of sites that sell posters to buyers. Ensure the authenticity of the site and the posters that they sell to prevent being swindled. You may also look for posters at garage sales and various cultural and art conventions, such as Comic-con.
Buying these posters alone is not enough. You need to preserve them safely. Never use heat or adhesives for mounting your posters on frames. Preferably use substances that do not contain acids. Store them away from direct sunlight and extreme temperatures. Earlier posters were folded and not rolled. So, if your poster is folded, then keep it so. Do not continually unfold it and refold it.
Also verify if the poster you are buying is a reissue or the original. Reissues usually have a R before the date.
It will be helpful if you do some basic research on this topic before you actually begin collecting.