That's a great question @maryfavour and congratulations for choosing a wonderful application - @what-app to ask this question. Delegated Steem Power is very valuable and a great way to push yourself up in the ladder.
There are two primary ways by which you can Lease or Buy Delegated Steem Power:
Lease Or Buy SP: A Case Study1)Go to https://www.minnowbooster.net/ which is a @buildteam initiative led by @thecryptodrive (Witness) @cryptomancer, @reggaemuffin (Witness) and their hard working team. There you can find an option to make a Lease Request. You just need to click that option, fill out the simple form about how much SP you want to lease and how much you are willing to pay and the duration of lease. Once you submit your request, you will have to wait for max. 3 days. If your request is not served by any investor within 3 days, your money will be refunded. Believe me it's very simple. I have written a detailed case study on this topic as well. Check this link -
2)Go to @blocktrades and buy SP for 3 months. As simple as that. Pay though your Steemit wallet or directly buy through any other crypto which is available to be exchanged on their platform. This is also the most underrated way to get the DSP.
I have tried both ways and I think you should try them as well. Maybe save some SBDs or STEEM and invest in both the platforms. I hope this answers your question. Let me know if you have any other doubts. As mentioned about you can read the detailed guide HERE