First you need to realize which sounds give you trouble. You can repeat all the alphabets and have a friend observe which ones you need to work with. You can also repeat a few words to see which combinations are difficult for you to pronounce.
If you don’t have a friend you can take help from or want to keep your therapy private, you can also use a recorder or install a recording app in your phone to record your speech. Just say all the words you find troubling and record them on the side. Once you are done with the recording, you can play your own pronunciations and compare them with those given in an online dictionary.
Drinking from a straw can give you practice to move your tongue back. Most cases of lisp are a result of incorrect tongue placement, and when you drink from a straw, your tongue moves back, which can correct the issue.
Practice speech using alliteration. This is a technique where a sentence is constructed using words which begin with the same alphabet or sound. For instance ‘Henry held a hammer in his hand”. You can work with alliterations that have sounds or letters you have trouble with and continue repeating them over the course of your day.
Try breathing through your nose, which can in a lot of cases reduce speech impediments. If you are a mouth breather, it might be one of the reasons behind your lisp. Just focus on breathing through your nose, and if you suffer from any related allergies, go to a room where external elements are not present and practice to see if this fixes your problem.
Whenever you speak, pay attention to your words and focus on the pronunciation you wish to deliver. If you find eye contact distracting, don’t look at people directly and try to simply focus on your speech. You can also try looking at the other person’s nose or mouth as you speak and not directly into his/her eyes.