You can start with nothing. By opening a Demo account, you can learn to trade without committing any real money to the markets.
Once you go to live trading, the smaller your account the longer it will take to achieve your ultimate goals. You can start with as little as £500-£1000, in your trading account, but your expectations and goals should reflect this.
If you are undercapitalised, you can look to get funded by a funded trader programme, but you must first learn the processes before you will be able to achieve this