That's a huge subject. There are lots of platforms and thousands of funds.
First of all if you are under 40 then you should look at a Lifetime ISA you get a credit from the taxman. I don't know a lot about them as I'm the wrong side of 40. If one isn't suitable then they money should certainly go in an S&S ISA so that all gains are tax free.
For a "platform", MSE did a recent review:
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Personally, I'm with HL. Their website is excellent but they aren't the absolute cheapest. Depends on what products and features you want.
For funds I'd start with something simple and low cost. As mentioned above a global tracker is a "fire and forget" type investment. Vanguard do som popular ones. Setup a direct debit every month and then get on with your life. Avoid the urge to check on it every day and play around with your portfolio.
Loads on information online such as YouTube. Just need to do some research.