There are some similarities and if you know Hindi (or Punjabi) you can probably understand the context of what is being spoken in Punjabi (or Hindi). However, you won't be able to understand every word of the other language. Also, the script is not the same. Again, there are some alphabets that are similar but you won't be able to read or write both languages just by knowing the script of one of them. Just to give a bit of a cultural perspective, in India almost everyone who speaks Punjabi is able to speak Hindi. However, the opposite is not true as Hindi is a much more widely spoken language.

Well clearly it is hard learning a new language and especially speaking fluently right away. I would say if you would really want to understand Punjabi then to learn a good amount of words and build from there. For example, if you are going to understand particular words that are being said in a movie, you would understand what is happening. I'm just not sure if you would like watching a movie if you couldn't completely understand what is happening. When I would watch Hindi movies, it really frustrated me at the beginning, but then I continued watching and eventually caught on.

There is a CLOSER relationship between hindi and urdu, except they use different writing systems