First up I’d say if this is new to you and you have £100 decide what you want to do. Is this the start of a long-term savings plan? Or a gamble and a short-term ‘punt’. If it’s a punt then you might risk bitcoin and Google blockchain info to get started. If it’s long-term savings I’d avoid digital currencies and think about the stock market.
In the old days you used to have to really know which individual shares you wanted to buy. And the problem with £100 is you’d spend £10 on transaction fees so you’d be 10% under water before you’d even started.
Today however there are low-cost easy ways to get started which use baskets of pre-assembled shares called funds. And these don’t have the same transaction fees as most places.
It sounds to me as though you could use a sense-check to get you into the right pool of investments. So something which is called a ‘risk profiler’. Some services require more than £100 to get going. So I would have a look at Wealthify or Wealthsimple with your first £100. If you haven’t invested already it usually makes sense to choose the ISA option for the investments as this will keep profits clear of the tax man, without locking your money away.