Most adults who intend to 'open a Stocks and Shares ISA when I get round to it' never do. The intention persists for years; the compound benefit lost is substantial. Today is as good a day as any. Twenty minutes online.
What to do specifically
Vanguard (cheapest for Vanguard funds), Trading 212 (commission-free), or AJ Bell (full-service). Open Stocks and Shares ISA. Set up monthly direct debit (£200-500/month sustainable for most). Buy global tracker (FTSE All World or similar).
Twenty minutes. Done. The compound effect from starting today vs 'someday' is significant.