I think that Peter Young's advice in Charge! to stick to one wargames period and make up imagi-nation armies for it is one of the most under-rated pieces of advice to a wargamer...although unhappily for me, I didn't read it when I was starting, or re-starting, in wargaming! But as advice for a busy person with a very full life outside of wargaming, it has lots to commend it. And it is perhaps still the most common way of wargaming, which I guess is to play Warhammer 40K and have done with it, for all one's hobby needs.
But for the more historically-inclined of us, my advice to myself might have been:
Always remember you are playing a game on a tabletop with models (or counters, or whatever); don't take it too seriously, don't overthink it, playing the game is the thing.
One period, many scenarios.
Build two armies, imagi-nations, which can be slowly added to as needed to play as many scenarios as you need to.
Make the scenario fit the models/toys.
Either build a 2mm/6mm version of the army for big battles and a 28mm (or above) force for skirmishes; or a single 15mm/20mm army for both. If blessed with a really big table, maybe consider 28mm (or above) for everything too. If cursed with a very small one, make the skirmish force in 15mm/20mm too.
Where you can, use 'generic' troops and vehicles that can then be given identities by use of appropriate command stands, separate flag bearers, that kind of thing.
If the one period has to be WW2 or later, one is faced with the issue of iconic equipment. In earlier periods, generally iconic equipment can be avoided but a Sherman and a Panther are signifiers different from each other in a way that a Marlburian Cuirassier and a Napoleonic Cuirassier are to my mind, not. In that case, pick Germans and a single Allied army, probably Soviets; or US or British/Commonwealth and Japanese, and maybe Germans too. I think there is a little less to be gained from making armies imagi-nations here although certainly not saying you couldn't do it. The issue with picking France, Italy, Poland and so on is of perhaps limiting your options a bit more than necessary - what are you going to use to recreate Kursk scenarios if your basic armies are Italians and Indians? Again, not saying you couldn't do it, just saying it makes more demands of your imagination. If the period is post WW2, pick one army with Communist Bloc equipment and one with NATO-Bloc equipment. Make these imagi-nations.
For all periods, include irregular forces alongside the regular troops: this really usefully expands the number of scenarios available.
If you absolutely must have more than one period, then three should suffice: an 'ancient' army (anything from 4000BC/BCE - 1600AD/CE; a 'horse-and-musket' army (anything from 1600AD/CE to 1880AD/CE); and one 'modern' army. There should definitely not be more than 5 (Ancient, Medieval, Pike & Shot, Horse & Musket, Modern)!! Fantasy and Science Fiction armies are perfectly adequate substitutes,
just take out the magical elements when using historical scenarios.
Really, the 'look' & 'fluff' of an army/period, whether historical
or not, should be the most important guide here: buy what you enjoy
painting the most.
Always consider expanding the dimensions of a force (e.g. sea and air, but also counter-insurgency, smuggling, policing etc. if that makes contextual sense) and the medical/engingeering/logistic/intelligence elements rather than expanding the number of forces. But expanding the number of forces for the same period is better than adding new periods (e.g. if you have French and British Napoleonic forces, add 1830 Algerians or Maharrata Indians rather than Zulu War).
Be reluctant to sell models, especially painted models. You bought them for a reason. That said, if you need the space back, either mentally or physically - or you need the money generally - then just do it - you can easily buy more figures when/if the time is right. Obviously none of this applies if you are deliberately painting and selling models as a financial sideline.
Other things to consider: the early wargamers used far fewer sets of rules and were more inclined to tinker with existing rules than buy more. There is probably something to this, since learning new rules does come with quite a time and intellectual overhead. So I would advise sticking to a set you like, especially for 'home' games and being quite reluctant to change - but if you must, then don't be afraid to do it. But don't just buy more and more rules.
Having a couple of how to make terrain books is useful, as are a couple of generic scenario books. Shop around for both to find the ones you like. The former may be less useful in the C21, although I still like having a reference book around as well as finding things on Youtube and blogs and so on. Specific period scenario books are great too, buy good ones when you see them. Have enough terrain, but not too much: consider having a make/buy as you need approach. This is reason enough to limit the number of scales/sizes of models you use.
Writing your own scenarios is great but can take a bit more time than you think to write good ones, so don't be afraid to go to your pool of generic scenarios. They still tend to be better than 'points-buy' scenarios or 'line the troops up' games.
Give thought on the practicalities of getting a game to the table: the board, the figures, the terrain, the rules, the scenario, dice, space, note-taking, transportation etc. This will reward you with many more, and better, games.
Campaigns are great but can be quite hard work. Lots of the 'greats' of wargames campaigning really aren't that amazing for actually getting a campaign on the table and played, they are more inspirational and thought-provoking. Campaigns based on existing boardgames tend to work much better, IMHO. If you do homebrew them, then simplicity and structure are your friends - the interesting situations and character of the campaign can emergence nicely from quite simple campain rules, whereas detail and administration will definitely make it harder work with little guarantee of a richer strategic experience.
If you are using wargames for serious research, I would ignore the figures and terrain and make DIY boardgames etc. The overhead is much smaller and you can concentrate on the design rather than the aesthetic and fun aspects.
Wargames magazines are generally pretty good but there is only so much brilliance to go around. One subscription at a time is more than enough for interest and inspiration, so unless you are playing an awful lot, I would keep it to that. On the other hand, wargames magazines considered in totality can satisfy the terrain, campaign and scenario needs. I would consider this as an either/or. There are some great blogs out there too which can satisy some, but probably not all, of these things. Taken as a strategic whole, they are great for modelling and painting advice, not bad for thinking about design, okay for free rules, not great for scenarios, and very poor for campaigns.
Anyway, I think this is how I would advise my beginning self - how about you?