Along my studies as an undergraduate I have written a short paper with some very neat results in. So I sent the paper to one of my professors to get some feedback. He replied saying that although some of the formulae I'd come up with are very pretty, it would be hard to get the paper published. He said I could try and send it to some journals but didn't really give me anything more to go on. Where do I go from here?
For some context on the paper, I have found numerous closed form expressions for infinite products similar to Wallis products and some more general formulae similar to Catalan-type infinite products.
Let me be honest: as an undergraduate, it seems very unlikely that you found any new and signicant enough result that can be published.
It sounds to me like that is what your professor was hinting at, although he may not have wanted to commit either way.
If you are intent on getting your paper published, you should first make sure that your results are new. That means reading books/articles dealing with similar topics, and asking experts if they are aware of such formulas. If the results are not new, they are not publishable.
Even if they are not written anywhere, it is also possible that experts are familiar with it and that people have just not bothered to publish them. Again, not publishable.
Finally, if the results are new you can try to send them to a journal. In that case you should try to find a journal which has published in the past papers similar to yours in content, topic and length. Again, if you cannot find such a journal then probably your results are not publishable.
If you do find a suitable journal, then you should type your paper in latex, in a format similar to their articles (abstract, introduction, statement of main results, proofs) and follow the guidelines for submission on their webpage.
PS: If it turns out your paper is not publishable, don't take it personnally. Most people get their first publishable results halfway through grad school.