Hey there!
Although I do have a tutorial on how to get mana and the best way to do so, I believe a more in-depth [and up to date!] explanation would be more qualified here. The best way to get mana is, by far, upgrading to slayer level 3 and killing the pigmen. You get 10 per every pigman kill, and hitting one will make them all swarm towards you, which makes it super easy to attack.
And if you have anti gravity, you can jump onto the acacia stairs decorating the outside, so you're too tall up to be hit, but you can still hit them. So start saving your money to buy heads off of /ah, or go to public warps [For Aztec, I recommend /warp pubg or /warp bestpubg, and for Oasis, I recommend /warp NL or /warp aquahub].
Sell what you don't need for money to start buying mana, as well. On Aztec, the price is about 20-30m per 1k, but I'm not too sure about oasis.
Good luck and happy trails! If you ever need help or a decent sword, message me in game [menaciingly] and I'll see what I can get for you! <3