Hello and welcome Greysword!
Some excellent advise is already given by Great Khan Artist and werennor.
For painting the figures, I'd recommend YouTube tutorials or written guides. For the Warriors of Rohan in the Pelennor Fields box set for example, I'd highly recommend this guide: https://www.hobbitarn.com/2018/08/29/ho ... -of-rohan/
For creating terrain, there are also many great guides on the web and YouTube. However, I personally still often use those DeAgostini magazines because they cover such a wide range of possible terrain, generally with cheap and easy to find materials. So I'd actually recommend picking those up on eBay or similar websites, since they are often not that expensive and often also contain narrative scenarios, which I find highly enjoyable. Once you have the basics down, it's easy to improvise and come up with creations of your own. That goes for creating terrain, but also for miniature painting.
If you don't have objections against buying terrain, you can easily find affordable and nice looking trees in most hobby stores (they are a pain to make yourself in my opinion). There is a wealth of great other types of terrain for purchase as well, though cost may vary greatly. My favourite store so far is Ziterdes (https://www.ziterdes.com/) and Thomarillion (https://www.thomarillion.de/?language=en&page=news). They carry very similar stock, I believe Ziterdes is a larger franchise while Thomarillion actually creates most of the terrain. Another great website is Amera Plastic Mouldings (http://www.amera.co.uk/?LMCL=f0n_Bc), which makes very cheap but effective terrain. TTCombat (https://ttcombat.com/) and Sarissa Precision (https://sarissa-precision.com/) make great terrain out of MDF.
Good luck and have fun! The Pelennor Fields starter box set is really wonderful and one of the best deals you'll ever find within this game.
