So I'm working on an altcoin for me and my friends to use. None of them are familiar with mining. If my computer is the only one mining, how much would that affect the economy? Also, how many transactions are needed for a block? If only 1 transaction per day is made, would the miner just not have any work? I forgot to mention the block reward is 10 coins.
Blocks can still be found by miners even if there are no (other) transactions. The reason for this is each block must contain a coinbase transaction, which rewards the miner with 10 coins.
(The above is assuming that your coin is sufficiently similar to something like Bitcoin or Litecoin. An altcoin you create could have completely different rules.)
As for how your mining could affect the economy, well obviously you would be the one ending up holding all the coins. If you're giving them to your friends to then trade, no problem. A few friends is of course not a very large economy.