Warm congratulations on your historic victory! For the first time in living memory, someone wholly outside the government "industry" has been elected President. You beat not just Hillary, which is the main reason I supported your campaign, but also the elitist, arrogant clique running both her Party and yours. And to cap it all you beat the media, for 80 major newspapers out of 90 endorsed your opponent. Well done, indeed!
Having won, you're hearing advice from a lot of people and this is to offer mine.
1. If you build that "wall", do it right. As in #2 below, I think there's a better fix for the very real problem of immigration, but if you do choose to solve it by having more control over the border, why not do it legally? Ever since 1789, the border has been policed illegally, so by making this change you can expose all your predecessors as scofflaws.
I refer to their disregard for the supreme law, the Fourth Amendment. It clearly says that government shall search nobody without a warrant, supported by an oath by a witness to a probable crime. Yet every year, about ten million US Citizens are stopped and searched as they re-enter the USA from abroad, often with extensive delays to their journeys, and without a single such warrant ever being produced!
Amendment 4 could very easily have been amended with a simple "border exception" - but it never has been! Instead, the CBP - the border police - pretend that the Supreme Court has made an exception of that kind, but as you know, no Court has any such power. Nor do you, and nor does the Congress. Only three quarters of the States can make any amendment to the charter under which you will shortly take office. Article 5 explains.
So, order CBP Director Kerlikowske to send all his staff home until the States have ratified a suitable amendment. It should take about a week. You'll then have drained the first corner of the big swamp.
2. Remove the motive to migrate - or most of it. I suggest that would be far simpler than trying to seal the border anyway. As you know, rational people sell their goods and labor where the price is highest, so low-skilled people in Mexico have a powerful motive to move North; the same work might earn them ten times the wage. The same applies in reverse to employers; rational ones recruit where the cost of labor is lowest, and government here has inflated that cost. To slow the flow, that differential has to be removed.
You'll preside over this country, not Mexico, so that means reducing labor costs here. At first sight that might seem unpopular, but notice: most of the "high" wage rate here is government mandated. You can remove some or all of those mandates, very few of which take the form of rewards the employee can spend.
Those mandates include minimum wage rates (often set by States, but from the White House you can use your influence) and work environment regulations that magnify the cost of hiring someone, and ballooning health insurance costs (see more below, in #3) and laws which give trade unions a heavy advantage in "negotiations." You know about that, from all your experience in the Trump Organization. Now, while Congress controls those laws, as President you can prevent them being enforced, for you are the Executive. If you tell your staff not to implement a certain law but no heed is taken, you can say "You're fired!"
Another strong motive to immigrate to the US is that one's homeland (in Syria etc) has been turned into a war zone. How so? - not exclusively, but largely by US intervention in the Mid East, and not just by Hillary Clinton while SoS. This has been going on for decades. Much of it has to do with support for the State of Israel, anathema to many Muslims. We both admire the way Jews have prospered since 1945, but using influence in D.C. to take resources from Americans and hand them to that State is not permitted by the Constitution. As President, you can end it.
3. Disentangle government from health care. That interference is what has sent costs way, way above what they would be in a free, competitive market. For over a century, the bill sold to Americans by politicians has been that "free" care is an "entitlement" - at the expense of nobody who is identified; presumably, the fairy godmother.
But for all that interference, the total cost of care today would approximate to the portion currently co-paid by the patient; 10%, 20% etc. This week I spoke to a cancer patient whose treatment includes a drug priced at $1,143 per pill. You know business; you know that in a competitive market that would be impossible. The government's FDA has made development costs so vast that pharmaceutical firms have to spend - and recover - many times what they would face without that agency breathing down their necks. The FDA will be under your control. You can fire it. There will be a Congressional fuss when you do, but voter feedback from those whose health care costs are plummeting will quieten any who screech for your impeachment.
Before government winkled its way into the industry in the late 19th Century, American physicians were delivering state of the art care for prices little more than trivial. They can do so again, if you move them back from the political to the market arena.
4. Close foreign bases. You've spoken of keeping a strong military, but under the charter that allows you to take office, there is no mandate at all for a standing army in peacetime, and its whole orientation was towards defense of the United States. We may debate whether a centrally controlled military is needed even for that, but the US has no business at all to police the world! As Commander in Chief, you can start the process of winding it all down by closing 130 foreign bases. They don't add to American security, but rather subtract from it, by providing trip wires and provocation, all over the world, and spreading hatred and fear of the United States.
If other countries wish to defend themselves, it's their affair. Not ours.
5. Welcome back the whistleblowers! Julian Assange, Bradley Manning and Edward Snowden are outstanding heroes who have exposed massive wrongdoing by government under your predecessors. They deserve a ticker-tape parade - and in Manning's case, a course of treatment in a top-class clinic for those with gender confusion, at the expense of the Clinton Foundation. You could do a deal with Hillary about that, see #6 below.
6. Prosecute her. But here's the twist: negotiate with Hillary now, this year, to get a full confession, a guilty plea, and an undertaking never again to support any political candidate in any way - and to finance Bradley Manning's treatment as in #5 above. In exchange you'd promise her that after conviction and sentence and after serving, say, a week behind bars in prison orange with cameras clicking, you will grant her a free pardon to prove you're not vindictive.
You may have noticed a theme in these suggestions: that government is at the root of every serious problem. That means America will be made "great again" only as government's role is reduced. As you may sense from the header of this page, I believe that is true all the way to a zero role. But every reduction helps. The best of luck to you!