If you got a diagnostic of 141 odds are if you do all the prep right you are probably capable of getting a 155-160, which will get you into a number of ABA schools. Something I see many incoming law students and incoming med students do is flip out about the LSAT/MCAT and expect to obtain near perfect scores. This however, is very unlikely and odds are you will not go to Harvard, Yale, Stanford law school. That does not mean you cannot have a successful legal career attending a law school that 98% of other practicing lawyers attended.
As for prep you can obtain materials from LSAC and order the old tests. Do as many practice questions as possible under strict time conditions. Practice makes perfect, but it will take a lot of time and effort to do 20-30 full length practice tests, but it is certainly doable.
As an additional FYI I do not believe any schools average the LSAT anymore so you can retake the LSAT if you do not do well without any consequence. Schools only have to report their highest LSAT so that is typically what they will do. A few schools might have exceptions, but I believe the vast majority of schools do not average the LSAT.
Continue doing well in undergrad and maybe get a few filler classes to get some free A's to boost your GPA, take the LSAT, then attend an LSAC forum to get admission waivers and talk to school representatives. Then start applying once your acceptances come in start reviewing this is a good article explaining how to choose a law school. http://www.legalmatch.com/choose-the-right-law-school.html