Alright. This is going to be terrible, but why not.
Let's start with tabs
Inventory
This is where any item you receive in the game will appear here (basically)
This stores the food you eat to heal, the armour/weapons you use in combat (until you equip them)
quests
This is a list of the quests you can complete throughout the game, of which completing can unlock new features, items, skills, etc. This will display what quests you have completed/incompleted and how many quest points you have (quest points are received for completing a quest. The amount of quest points you receive throughout a quest varies depending on which quest you complete)
skills tab
This is a list of the skills throughout the game. You can level these up by performing certain actions, let's start with the basics. Combat. Basic combat skills are: Attack, Range, Magic, Defence, Strength, Constitution. Of course, if you equip a bow and arrow and attack something, this will result in giving you ranged experience (experience; what levels you up throughout the game), if you equip a staff or just cast a spell (discuss how to later) you'll receive magic experience. If you have a melee weapon equipped, depending on which attack style you choose (discuss how to later) it will train either Strength, Attack or Defence. Now with the non combat skills (mining, fishing, woodcutting) a few basic ones to get started. It's as simple as grab a fishing net, or a hatchet (bronze or iron hatchet at level 1) and either finding a tree to cut, find a fishing spot or find an ore to mine. Throughout levelling these skills, the higher the level, the more options of ore, wood or fish you can obtain.
task list
Not something to worry about too early in the game, you'll find out more about this later on.
combat
This will display your combat level (you receive a combat level after a certain amount of times you level your combat skills) this also displays what type of "attack style" you are using. Discussing from earlier on where different styles train different skills, the attack style "aggressive" will train strength, the attack style "accurate" will train attack and the attack style "defensive" will train defence.
equipment
This is where anything you equip goes. Armour, weapons, etc.
This will also display the "bonus" your armour/weapons give when you click one of the buttons down bottom, it also displays the items you will keep if you happen to die throughout the game.
prayer
This is your prayer tab, you will unlock new prayers when you level up your prayer skill (this can be levelled up by burying bones) different prayers will have different affects. Just hover your cursor over the prayer you wish to examine and it will tell you what that affect has.
spellbook
This is the spells you can cast throughout using magic, the higher your magic skill is (levelled by training) the more spells you can unlock. There are different sorts of spells, some do damage to opponents, some prevent your opponent from moving and some can weaken them. Spells can also teleport you to different locations on Runescape.
friends/ignore list
If you meet someone you like, add them to your friends list so you can have knowledge of whenever they're online and what world they're in. You can add people to your ignore list that you don't like, this will prevent you being able to see messages they say.
friends chat
This is a chat room owned by a certain person (the persons name you type in) you can open this tab, type someones name and join their "friends chat" people commonly use this for gambling. (not too important)
clan chat
Throughout the game you might eventually join a clan, this is the chat channel you join upon joining the clan.
options
this is where you can change resolution, sound and other tools also.
emotes
These are something you can perform. You can unlock more emotes by achieving the highest level of a skill, completing a quest, or participating in an event
music
When you travel throughout Runescape, you will unlock different soundtracks. This will show you which soundtracks you've unlocked (green) to those you haven't (red) this will also display the amount of songs you've unlocked.
journal
Just a way to leave notes to yourself or reminders to complete certain things.
Now that tabs are covered, let's move on.
Grand Exchange: A place in Varrock (located on map) click the map/globe icon above tabs.
This is a place where you can buy and sell items. [Quite important]
Getting familiar with locations will come in handy, especially if you die and need to return to your gravestone (once you die in Runescape you may lose items, but luckily you can run back to your gravestone (placed upon dying) and pick up any lost items)
A good start to making money is find the farm across from Lumbridge [starting location] and killing chickens, this will gain you a few levels, but also pick up their feathers and/or bones.
Hopefully this is a good beginning, anyone can patch up on what I haven't covered.