adding the_excerpt function

Hi, I am new to WP and learning as I go, because I had to put together the web page for my work. So please bare with me because I am a rookie at best! I am using Pluto theme and I need to change the amount of characters the excerpt has on the main page. It doesn’t look right. I was able to establish PLUTO does not have the function. I installed a couple excerpt plug ins and when they didn’t work I kept looking and found out that if it didn’t work the theme probably didn′t have it and honestly I havent been able to find it. Is there a way I can add it, where and how? THANKS!