It takes time — and money — to diversify a portfolio, since your goal is to spread your assets among a variety of different investments. Unless you have a lump sum to invest, you'll have to accumulate the money to pay for each new investment you want to make.
That's one reason why people who want to diversify, but have a limited amount to invest, may choose mutual funds. All funds own a number of investments, and some funds spread their investments broadly within an asset class, owning stocks or bonds of different sized companies in different industries or sectors.
Provided the fund isn't too narrowly focused, it may provide you with ready-made diversification.
Don Kittell