Investment
The reason I say investment is because not that many people know about investing or what type of platforms they can trust to help them to start investing, before I had started investing I kept coming onto sites which talked about needing a broker to do the buying and selling for the stocks you may need from a company.
In regards to investing i would recommend starting out with an investment platform called wealthify if you live in the UK. This will allow you to invest in the stock market with as little as £1 and the company will invest in funds on your behalf. This is helpful when you are first starting out as you dont know what to invest in as wealthify gives you articles to read on the current stock market, how it is performing and gives you news in relation to the stock market. Reading these can be beneficial as they also give you insight into what ETFs are and how to pick stocks to invest in. However, their fees are quite higher than other platforms. You can find some images for how it would look inside Wealthify.
After using a platform like wealthify or if you know what investing in the stock market is like and what you want to invest in, I would recommend 2 platforms to move to if you arent happy with the performance of wealthify or want to choose what you invest in, Trading 212 or Vanguard. Trading 212 lets you invest in funds or stocks for free, from as little as £1, with the only charge being the OCF (on-going charge fee) which is the charge for holding onto the stock that you purchased. Vanguard is a platform that lets you invest only in vanguard funds but you have to supply a minimum of £500 as a lump sum or £100 every month.
I would recommend that if you are confident in your ability to choose stocks or ETFs or if you dont have alot of cash to hand, use Trading 212 as it is more convenient and allows you to invest more than £1 in any stock or ETF you want (which is offered by the platform) and gives you an app to monitor and buy stocks or ETFs in your S&S ISA from your phone. If you want to invest souly in Vanguard funds, want to invest in funds autonomously (so you dont choose what stock to pick you just have a direct debit that takes money from your account and invests in the fund of your choice on the platform) such as a "set it and forget it" mentality and the funds you want to invest in aren't available on Trading 212, I would then recommend investing in Vanguard directly.
If you live in the US then most of this information doesnt apply to you. I would recommend opening a Roth IRA with Vanguard but this is talked about in more detail by Nate O'Brien on Youtube, please check him out for more details.
Update; so with Trading 212 you are able to buy a vareity of shares and ETF's that they have to offer either as a full amount if you have the funds or like myself where you basically buy fractional amounts and over time buy more or sell the shares. They also tell give you a pie chart to show which share or ETF you have the most in and how much money you have lost or earned from a day all the way to 1 year. Do keep in mind any money you lose is your own responsibility, which is why it is best to have your investments spread out across different areas. Trading 212 does also provide videos within the app and on youtube to how to use the interface, use other features and explain different terms and strategies used.
Best thing about the app is before using actual money you have the option of using their practice version which uses real time share prices so you can practice owning and selling shares, then once you are ready switch to a actual account wheather that be trading212 invest or trading212ISA you can put what you have learnt into actual trading.
2021 update:
Trading212 is still a good application to use if you want to manage your investments yourself and have other featurs builtin like auto pie where it invests into different companies for you.

