I wouldn't use an IFA, but then I was badly advised by one years ago and don't trust them at all.
Before investing in the stock market, make sure you have some easy access savings too, minimum 6mths living costs. Yes, interest rates are crap, but you do need them.
We have been investing in stocks and shares ISAs and SIPPs for many years, all DIY. We made 1 or 2 mistakes initially (see my advise below) but we have made a lot of money from them overall.
We use an online broker called Hargreaves Lansdown, who have a very good online platform but aren't the cheapest fees-wise. You could maybe look at HL for investment ideas (they have a comprehensive list of funds and website easy to use) then look at other brokers for comparison costs.
Have a look at this website for advice and comparison of online brokers.
monevator.com/category/investing/passive-investing-investing/
There are many different funds you can invest in, plus each broker will also offer some tracker funds with varying degrees of risk.
You can also buy shares in individual companies, but you really need a crystal ball to know which will be a success. DH bought Amazon many years ago and has made a bomb (on paper at least!), but others he has lost on! I prefer to stick to funds only.
The main advise I would give is:
Split between ISAs and SIPPs in both names
Whilst former doesn't have tax advantages, it has flexibility on when you access the money compared to the SIPP and can be withdrawn tax free. You never know what life events may happen and you want access to money, which wouldn't be possible if all locked into a SIPP.
Split the money into more than one fund, ie diversify, to spread the risk.
We have UK funds, European, US, Russian, Far east and a number of speciality funds too. But some UK and European funds are probably good as starters. (The HL website allows you to see top 10 companies that each fund invests in)
We made the mistake of not diversifying in first year or 2 of investing in ISAs, in late 90s, and whilst some funds increased spectacularly, I was burnt with a technology fund in dotcom crash!
Drip feed the money monthly, so that you even out the peaks and troughs of the stock market.
Again, we didn't do that initially, we invested full allowance in April of each new tax year and quickly learnt it was the wrong thing to do. If you invest all money on same day into a stock whose price is in a trough, all your money will grow, if the stock price is at a peak, it will drop - the difficulty is knowing whether it is in a peak or trough on that particular day. By drip feeding monthly, you even the risk out.
If you need a 'home' for the money whilst you wait to drip feed it, then look at 1, 2 and 3 year savings accounts. They tend to have higher interest rates. My DC both have an inheritence which they are drip feeding into ISAs. DS (20) has his spread across a number of 1 & 2 yr accounts, to access in March of each year then transfer into easy access a/c to drip feed throughout following tax year.