It would help us help you if you could elaborate a bit on how the "upload limit" is impacting your ability to use your site "to the best of its ability". I assume you are talking about the PHP max upload file size; depending upon what you are trying to do, PHP may not even be the best way to accomplish that.
The link that DH provided is a good article on what is probably the easiest way to modify the characteristics of PHP on Dreamhost, but that is not to say that is is "trivial" for those who are not reasonably comfortable using the shell. The page itself "warns" of this:
Frankly, the article does a very good job of describing how to accomplish that task, if you have the requisite understanding of what's involved. The problem with attempting such changes without the requisite knowledge, is that if this is done incorrectly, you can re-introduce security vulnerabilities and resource usage into your site that have been purposely "engineered out" of common use by Dreamhost to insure the maximum safety and efficiency of the server to protect others who share your server.
Doing this takes upon yourself a huge responsibility to be sure your actions do not adversely impact others on your server; hence it should not be undertaken until you thoroughly understand all the potential ramifications (both from the security and resource usage standpoints) of what you are attempting to do.
That said, and given your own description of your knowledge and skills at this point, it may well be there is another "simpler" and "safer" approach to take toward your development problem, which is was the point of my intial paragraph in this reply.
If you tell us more about what you are trying to do, it will help us evaluate whether there is a better approach to take than customizing your version of PHP. If customizing PHP is, in fact, the best, or only, way to solve your problem, then we can be of help in pointing you to the correct resources to aquire the understanding you need to complete the process and/or help you with specific details as needed.