1. Do i do the same and open various wallets and buy the coins that way or do I have to use an exchange like Coinbase or Binance to buy the coins?
You can buy crypto from centralized exchanges like Coinbase or Binance, you can also buy coins from decentralized exchanges like Bisq. To learn more about centralized and decentralized exchanges, read this:
If you’re buying from exchanges, make sure to move your coins to a non custodial wallet after purchase.
Read this:
Reminder: do not keep your money in online accounts2. If/when I want to sell the coins do i withdraw via the wallets or do i go onto an exchange and sell? Then transfer the money to my bank account.
Like I said before, if you’re hodling your coins for long term, it’s recommended that you store your coins in a non custodial wallet. You can send the amount you want to sell to your exchange account or sell directly through p2p.
3. Is there a secure wallet I can get with can hold all the coins in 1 place or do i have to open multiple wallets?
Unstoppable wallet.
I would like to buy and hold the coins long term but would also like to sell short term if I don't feel i have chosen the correct coin/s.
Investing in altcoins is very risky, I’d suggest you stick to Bitcoin and Eth for now.