first thing to do is assess your risk tolerance and goals.
how comfortable are you with seeing your portfolio go up and down in value? how will you decide when to get out of a stock/ETF that is not performing well and losing you money?
stocks as a whole are riskier than ETFs however you have more control over them and can choose specific companies you know rather than just blindly buying a chunk of the whole market.
on the whole, for long term investors, dividend paying companies perform better than non-payers that can only get you returns when you sell. for those with little risk tolerance or time, ETFs will offer a stable growth and source of income.
using my own portfolio as an example, I have put just over 50% of my portfolio into purchasing GSK shares. from what I can tell the business is solid, it has global reach, it has a very large market cap and share liquidity, additionally it is currently at the low-price end of its spectrum so the dividend returns are roughly 5.6% annually which would be paid out in part every quarter (3 months). it is also a rather stable stock with a low volatility beta which makes it less worrisome to invest. I plan to hold these shares until the price is at least £16 per share, collecting the dividends along the way. this means I can leave those shares alone and just check up every now and then.
when people tell you to diversify, the important part is to not have all your funds in 1 basket, get a few different shares across the industries you are familiar, this will reduce your risk profile and opens up the potential that while 1 of your positions is doing poorly due to the market influence, another position may be doing well and reduces the volatility in your portfolio.
as a beginner, I recommend getting no more than a handful of shares, either stocks or ETFs as you will want to be very familiar with them and have an easier time assessing your options. keep educating yourself in how the market works and research as much news and financials as you can for the companies or funds you decide to invest with. being on top of the news and fundamentals will be your best shot at securing your profits or getting out before a major loss that can take months to recover.