Do I need to maintain certain amount of balance in order to keep my account active?
For the first question, it depends on what you mean with "active". This term has at least two meanings in the context of Tezos. First, an implicit account (i.e. a Tezos account that is not associated with a smart contract: a regular account that is used to store and transfer tez) is either present or not in the context. The context contains a mapping from accounts (represented by their address) to their balance. If the balance of an account falls to 0 mutez, then it is removed from the context. However, this does not mean that the account is no longer usable. If someone sends a transaction to the account, increasing its balance from 0 mutez, then it is re-introduced in the context. Note though, that each such addition to the context has a "origination fee", currently set to 0.257 tez. This fee is payed by the sender of the transaction.
This is different from what the Tezos whitepaper, section 3.3.2, states:
A contract needs a minimum balance of 1 to remain active. If the balance falls below this number, the contract is destroyed.
This has since been changed.
There is another sense to the word "active", that relates to baking. A baker runs a node that validate blocks on the Tezos blockchain, and receives rewards for this service. If a baker is not reactive, it can become "deactivated": it will no longer be selected for baking and thus forgoes any potential rewards. You can read more about this here.
The simplest way to start trading, if you do not yourself want to run a node and handle a wallet, is to trade using a crypto exchange such as Kraken, Coinbase, MXC or Binance. You can find a more complete list of exchanges here.