Is it possible for someone to theoretically take over the bitcoin network and create their own Bitcoins if they own 100% of the hash power?

To mine on a Mac you will need MacMiner: http://macminer.fabulouspanda.com/
MacMiner can mine all SHA-256 coins and all Scrypt coins.
NOTE: MINING ON A MAC IS VERY STRESSING FOR THE COMPUTER, I AM NOT RESPONSIBLE IF IT DIES

To mine on a Windows you will need GUIMiner for  Bitcoin and GUIMiner-Scrypt for Altcoins: https://guiminer.org/ and https://guiminer.org/guiminer-scrypt.html
GUIMiner supports all CPUs.
If you do not have a AMD or NVIDIA GPU you will need MultiMiner: http://www.multiminerapp.com/ (this can also be used if you have a AMD or NVIDIA GPU.)

You will then need to find a mining pool, below are mining pools I recommend

Bitcoin: https://slushpool.com/home/
Litecoin: https://www.litecoinpool.org/
Dogecoin: https://aikapool.com/doge/index.php
Monero: https://moneropool.com/
Peercoin: https://peercoin.ecoining.com/
Zcash mining pool: https://zcash.nicehash.com/
Ethereum mining pool: http://ethpool.org/
Dash mining pool: https://dash.suprnova.cc/
Gridcoin mining pool: http://pool.gridcoin.co/

After that, you will need to configure your miner

To configure MacMiner click on the cog button that says "General" underneath it, select the coin you want to mine and input the details the pool gives you once you sign up and create a worker. After that, click "save" and "start".

NOTE: For some Windows graphics cards you may need to install OpenCL, search on google how to do this for your graphics card as it is different for each graphics card.

GUIMiner and GUIMiner-scrypt will open to a screen that asks you to put in the pool details, select coin (if you are using GUIMiner-scrypt) and put in your worker details. It will also ask you to select graphics card / CPU you want to use for mining. Once you have filled in these settings, you can click "Start Mining!".

Good luck with your mining!

You can't cut blocks off and still have your wallet work until the whole blockchain decides they want to. Satoshi detailed about a way to cut blocks off without in-validating the blocks after them, but doing this would require most people to do it.

Someone correct me if I'm wrong, but this is my understanding.

Try blocking the IP in your router's settings. If you use a hosting company they should have this in your control panel.

Step 1: Run an anti-virus
Step 2: Never send coins to people claiming they will 100x your bitcoins
Step 3: Never download shady files
Step 4: Cry about your lost bitcoins.

How much did you loose?

You could try sending a small amount to that address and leave a public note asking them to give back your coins, they probably won't but it's worth a try.
You could also go to blockchain.info and check the IP Address the transaction was sent from and try and trace it, this won't work if its an electrum or public IP Address. It might also not work since that's not necessarily the source of the transaction.

It will be hard for ethereum to do so as bitcoin has sort of created a monopoly (it's top of the market). Bitcoin also has significantly more users. Ethereum will probably become the main altcoin.