GPs aren't involved in assessing SpLDs (mine explicitly said this to me) so you need to do it through your school/college/university normally.
Your school should have a process for screening for SpLDs and then assessing them if in the screening you are identified as likely to have it. You'll then have an assessment and they will determine if you have a SpLD. Once you get their report back confirming the diagnosis, your school will be able to put in place various support mechanisms as needed.
In terms of what support you'll get this may vary depending on the school, your studies, and your SpLD. You may just be entitled to extra time in the end but they may recommend some additional support options.