I think many new vegans over-complicate the nutrition thing. However, it is also true that many new vegans are too ignorant.
I think the very first step you should take is to get a physical at the next opportunity. If you have had one in the last year just call your doctor and ask him to email the results.
Nowadays most blood tests are done in groups. And the common issues like D, B12, etc. don't have to request separately. they are all included. This will do two good things. You can see if you had or have any issues. and also it will be a good baseline for you to evaluate the results after being vegan for a while. Except for B12 a lot of supplements aren't even necessary. The vitamin and supplement industry is a multi-billion dollar industry and their ads and stuff can be misleading.
A lot of people, even non-vegans are deficient in D. Especially in the winter and in the northern latitudes. Also because no one goes out without hats and sunscreen any more D deficiency is getting more common. But plant milks are usually supplemented with D (and calcium and B12). And multi vitamins also will have D (and b12).
Besides the physical, the next best thing to do is start a Cronometer account. It's free. I'll include a link to the tutorial at the bottom. You plug in the food you eat and it shows you with colored bar graphs and pie charts how your nutrition is. So at a glance, you can see if you really do need more B, or D, or iron or what. And if you need more fat or protein or calories, too.
In most cases, it's not too bad to go over 100% of the RDA. but there are a few notable exceptions. Calcium is one. Too much calcium can lead to hardening of the arteries. Iron can be a little tricky. You might be getting 100% of the RDA and still not get enough because your body isn't absorbing it all. But you don't have to worry about too much iron if you get almost all your iron from food. If you eat too much iron your body will excrete it. And your poop will be black and rock hard. So you actually have a built-in warning light for too much iron.
The only other supplement that may be necessary is the Omegas. We have already discussed them in a couple of other threads so I won't get into it again here.
As far as food goes, you might want to get Dr. Gregar's Checklist. Either print it out or get the app. It is a really good guideline on what to eat each day.
Speaking of Dr. Gregar he has written the book How Not To Die
which is one of the best vegan nutrition books. he also has his own website and youtube channel.