Martinod buying shares is, to all intents and purposes gambling. Shares have in the past trended up but we have had the banking crisis and COVID within twelve years of each other so you can certainly lose money, it’s not a one way street.
Hargreaves Lansdown are the brokers I use and you can check out the costs of their services on-line.
If I remember correctly you can have an isa where you choose your own companies to invest it used to be called a self select ISA , I think.
I however invest in funds, mostly income and high income funds. This is where you let the fund manager choose the funds he thinks will achieve the aim of the fund. In my case an income.
These funds allow you to invest modest amounts every month so they may suit your investment interests.