Air not blowing as strong

This summer I have noticed that my A/C is not blowing as strong in my van. It is still cold, it just seems weaker. I used to hear a little squealing sound sometimes when I would turn the air on. It was occasional though, not every time. I am not sure what to do but I had a really bad experience with a mechanic on another car recently so I am nervous to take it in. My dad is somewhat handy with cars and can do many things with them that do not require really unique or special tools. Is there anything we could check ourselves or am I destined for a mechanic? Thanks in advance for your help.