The game gives your a small tutorial to introduce you into the game and learn some of the skills.
You do not need membership right off the bat, membership will be a better benefit when you have higher skill levels.
There is no true point to the game, you can do some quest to follow the story line of Sliske and the new god wars but that will require a bit of time as you need to train the required skills and do the previous quests.
The General Exchange (GE) is a great place to sell your items, As a free player you will be given 3 slots they stay in use until your item is sold/bought.
Do not buy an Iron weapon/armor they are not worth it as a few levels latter you will hit steel. Then you can go up to Black then Mithril to Adamant and finally to rune. When member you can go to dragon.
A good source of money and combat xp is to kill cows or chickens. Cows drop cow hide and meat that you can sell for cash. Or tan the hides at a tanner and sell even more.
Chickens drop Feathers and raw chicken, the feather stack and sell for a good price as they are needed for some fish and fetching.
If you feel like you need alot of money you can look up Free Guides online, or Mine or Woodcut and sell the resources.
And watch out for scammers, with the trade window it shows the GE value of the item being traded so they cannot overly scam you.
Nobody can trim armor (yes still out there) trim armor is just a cosmetic look and offers no combat befits.
If you are stuck or in need of help going to the GE and asking will be a better chance of finding help.
Look at the world map and learn the world, One of the first cities you want to get to is Varrock as that is where the GE is located.
Lodestones are a great source of transportation and each major city have one including varrock.
Almost all skills will use material from the gathering skills, like Firemaking will use the wood for woodcutting if you plan to train them all maybe keep the resources instead of selling.
That should be a good start. Runescape is an older game but it is still a good one. Admittedly it is a grindy game you will find that out as you get higher levels of skills.