Take the "poker courses" on this site, they're good.

And studying is not optional. Sorry, but it be truth. Especially on Stars, NL25 is extremely reg-heavy, and they will eat you alive. I honestly have no idea why people play there, there's much easier places to play. (As you know already.)

I would aggree with CV, studying and off table work is not really an option. I know when I get in a poker playing groove and go days without studying that there are times when my game slowly starts to deteriorate. Usually culminating in one day where I get absolutely crushed and the wind taken out of my sails. After that day I usually take a day off and watch a bunch of vids and do some off table work on ranges, pre and post, then get back to the tables to find that I am playing well again. I end up playing more confident and making better decisions and I don't think that is a coincidence.

Its in Jared Tendlers book, The Mental Game of Poker. Its actually a form of tilt when we don't do off table work and see our game start to fail and then cram all of our study into one day. He recommends doing just 15 min a day. I think it is where @CrazyCookie came up with his video series about being able to study in just 15 min per day. It keeps our mind sharp and allows us to continue learning and moving knowledge from stuff we have to think about to do, Conscious Competence, to stuff that becomes automatic, that we no longer have to think about to do at the table, Unconscious Competence.

I know this can be time consuming, but check the forums. There are a ton of threads about pre flop ranges in one form or another. People asking about things like SB 3betting ranges or BU 3bet defending ranges, etc, etc. Search for any responses by @eroticjesus He takes a very mathematical approach to constructing his ranges and makes some very good responses on how to construct your ranges, so that mathematically they are very hard to exploit, if that is what you want to achieve. Remember that pre flop ranges have to be fluid to some degree as there are a lot of factors to consider when deciding which hole cards to play. For example, opening A2o-A8o from the CO in an unopened pot may not be standard, but maybe you have A7o in the CO and look and see that you have a couple fish in the blinds and decide that opening it now could actually be profitable because those fish are going to make a lot of mistakes post flop.

I would get out a pen and paper, or a word document and watch some vids. This is something I have been meaning to do. Don't just watch the vids and see what happens. Watch coaches like @Komododragonjesus and actually take note of all the hands they are opening, defending, 3betting, etc from the different positions. You can probably construct an entire set of ranges that feel comfortable to you just by watching a few of the coaches and seeing what they do and listening to what they say when they do what they do. One of them may open a hand and say something like this is probably the bottom of my range in this spot. Well now you know those hole cards are the worse that they will open there. Lets say that it is K7s, so now you now any Kxs hand bigger than K7s is an open for them in that spot.

Laslty, you will have to test out the ranges and see how you feel. I got a set of ranges from someone and whenever I used them there were times when I just didn't feel comfortable opening or 3betting certain hands so I adjusted them to where I feel comfortable. If you have HM2 you can actually filter for hole cards and set filters to see how you are actually doing with all the different hands in the hand matrix. A simple example is, go to more filters, then position and action, set hero's position to whatever you want, ie EP, then set raiser's position to the same, EP and click ok. Do this in the stakes tab of HM2 and try and have as many hands as possible. Then on the bar between the stats and the actual hands there is a colored grid icon, click on that and it will show you how many BB/100 you have won or lost and on how many hands in a color coded hand matrix so you can start to get a visual representation of how your range is doing over all under the set criteria.

This is an example for EP open on BV , so it is showing my results for 716 EP opens. Its not a lot of opens so there is still going to be variance in there but you can start to see the edges of where I can go, as far as opening. Not sure how K7o got in there, must have been a misclick limp or something. LOL!!