If you search the splinterlands community for post by Hive account@quixoticflux you'll see some really good guides for beginners broken down by splinter. Best advice is to choose 2 splinters you like to play, and focus on getting as strong as you can in those splinters. The above mentioned guides can help. For beginners, Fire is a really good splinter to invest in. After that, Earth, Life, and Water are popular choices depending on your play style.

For free to play, your progression will be fairly slow. Re-invest your DEC in rentals according to the above guides. Do your dailies and claim those rewards religiously. Sometimes you get lucky with a big reward. For just a little investment in the game beyond the spellbook, the rental market opens up a lot of opportunities for you to build those first 2 splinters.