I started playing splinterlands over a month ago and honestly did some many mistakes. Up until now, I still have much to learn in this game and a lot of things I have no idea of. But playing in a month now also gives me some useful ideas, tips and strategy which help me a lot to earn profit in this game. The first two weeks I play in this game were almost a profit loss. The summary of mistakes I did when I was new in this game is:
-Burning up my ECR or Energy Capture Rate which consumes everytime you play the game. It only regenerates 25% per day.
-Playing the game by not joining any official Community Group, Discord, Telegram etc. Having groups helps you a lot especially in a newbie which you will have a lot of questions in game. The advantage of joining a group is that all experienced player will help you answer all your query, problems and questions.
-Renting a lot of cards to get power but in the end some of the rented cards is not suitable to your other rented cards. Also the DEC I rented is much more than I gained, so basically I did not gain any profit from
it.
-I bought Question for 750 DEC for 5 Loot Chests in addition to the Daily Quest Reward Chest. Its like a gamble but it is not recommended if you dont want to loss some DEC just like me. Because the rewards I looted in that chest only cost below 100 DEC which then again a waste of my DEC.
-Be part of the community. Joining Discord, Telegram and any other social platforms which give you knowledge about the game and they can help you answer your questions and queries.
-Posting in Splintertalk, Hiveblog and Peakd. Write any content related to splinterlands and just put a tag or keyword #spt and #splinterlands and earn rewards depending on your content that could attract the community especially 
-Control your ECR or Energy Capture Rate and dont burn it so much because if you have below 50% ECR, that means that you only gain half of the DEC reward in that game. My strategy in DEC is I play until 70-75% ECR so It will regen 90-100% the next day. This is not a pro advice but it is an advice I get from the experienced player in the discord community. But it is up to you how you handle your DEC.
-Effecient Renting. Renting a card that could help you ranked up in higher leagues because in higher leagues, more chest rewards and more DEC given after you win a game. But make sure your DEC spent from renting is lesser than your DEC gained to make profit. Another thing to consider when renting a card, is rent Gold Foil Cards, Alpha and Promo Cards which gives you additional bonuses in winning a game.
-Know your Summoners Level and Abilities. It is very important to research what summoner will you rent/buy to get you in higher ranks, because some summoners have advantage or disadvantage to others. So do your research and reach out to some experienced guys in the community. They will help you.
Final thoughts, play and try the game so you can experience the game because experience is the best teacher. You will learn gradually about positioning of monster and selection of additional monster, summoners etc. You will also know or discover your own meta deck to counter any other summoners. So, play splinterlands and do get experience and feel the thrill of this addictive game.
Official Website: https://splinterlands.com/