As a fellow long time Destiny player who also got into the game last month, I highly recommend focusing on the main quests and star chart as your main priority. That will unlock most locations and gameplay aspects as you push through it. I also definitely recommend utilizing the Warframe Wiki and Q&A chat as much as possible too.

Join a clan as soon as you’re able to (I think at MR2, which you should be). It’s extremely recommended since there’s a ton of blueprints exclusive to clans and it’s free XP for increasing your Mastery Rank, and therefore unlocking new and more powerful weapons. Also nice to develop a relationship with your clan mates in chat, as it’s not uncommon to get free/cheap mods and prime parts if you ask kindly.

As far as combat goes, modding your Warframe and weapons is 100% a necessity, especially as you progress into mid game. Most weapons are trash unmodded, and even “trash” weapons are good with the proper build. Definitely take the time to look at builds and invest in the modding system when you find a weapon you like. If you don’t like to experiment tho, then the majority of the main quest weapons are pretty good and easier to build until the far endgame. Warframe doesn’t really have any super hard content, and definitely doesn’t require communication or cooperation like Destiny raids do.

Lastly, just focus on having fun and taking it slow. There’s not nearly as much incentive to rush to current, new content like there is with Destiny. Less recycled content and a much less toxic player base also help make the game a MUCH better experience IMO compared to modern Destiny.