Now I am not the best player on DOTD, but I can give some tips to newcomers. First I SUGGEST that you buy land to get some income. I didn't do this and now  I have to wait awhile for some money to get the gear I need for missions.
Second Try to go to the bottom of the quest page and just buy all the gear that you need for that realm.
Third Always participate in raids. The loot and EXP that come with the monsters are always worth putting time into. PVP is also something that you want to pay attention to. The victory tokens that you get from winning allow you to get stat points, which are VERY important.
Fourth Pay attention to crafting. In every realm you will get a special item that will allow you to craft stat points. Crafting also allows you to upgrade items, or create them. (derp derp)
Fifth Spend some stat points on Energy and Stamina. Energy lets you do more missions, and stamina lets you attack more and do 1, 5, and 20 attacks on raids. (I suggest you get at least 20 before laying off of stamina.)
Sixth Go to your Citadel. The Citadel lets you make troops, make armor, make weapons, get mounts, make potions, get pets, learn legions, and upgrade your Citadel buildings.
Seventh Don't spend your planet coins (unless you buy yours) on energy or stamina. Planet coins are a good way to get really nice gear, and you don't want to waste them just buying something that you can wait for.
Well that's all for now. Let me know if i missed something in the comments.
Have a nice day!  :D