| Start exploring internships in the fall. The very good students secure one in the fall. That's also when companies come to campus to recruit.
Usually, winter is when the process starts. If you apply online, winter is the time the postings start to be placed on the job application board. It gets updated frequently starting from January, and hiring is sometimes on a rolling basis.
Most of the interview that I have seen take place from late February to early May. It is very difficult to get an internship after your freshman year unless you are either really good (have special skills that the company likes) and/or you have extreme passion for doing the work for the company you are applying for. An average freshman student usually doesn't have enough theory knowledge to perform many tasks anyways. So try to be better than average, develop some interests, and pursue something that matches you.
Most people I know receive internships after their 2nd or 3rd year. A lot of ME job requirements require completion of 2 years of course works anyways. Still with that prerequisite, it depends on what you have learned on your own, as sometimes your undergraduate curriculum doesn't cover the skills that the company wants. For example, some companies interested in MEs want knowledge in MATLAB, Pro-E, FEA, Autodesk, Catia ... and I haven't had any exposure to that yet.
Probably answered parts of your question. I would just advise you to be flexible. If you change your mind about your major, there will still be internship opportunities as long as your qualities match what the companies are looking for. If you don't receive one by your rising senior year, then you have to start to panic. | 
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