It doesn't matter where you go. What matters is, you go and learn from your experience and go again and again to tell people the exact same thing that I'm telling you right now.
P.S. Before my first solo trip I went solo to a pub :p P.P.S. This blog might help you:
https://www.gonimad.com/blog/orchestrate-your-solo-travel
If you are from Delhi or near you can go to following places
Jibhi, Jalori, Teerthan, Kasol, kheerganga, parashar lake, mclodganj in Himachal
In Uttarakhand- auli, landsdowne, Harsel,chopta, Rishikesh, Dehradun.
If you are from Mumbai or around- Goa, Pune, lonavala,gokarna,Kerala, Andaman, Pondicherry
For a first solo trip i would suggest you UTTARAKHAND is one of the best destinations. Lovely weather specially in summer, sacred temples, beautiful greenery, hilly routes, cultural diversity and smiling face of people wherever you go.
Start small somewhere close to your location then build up the confidence and courage for bigger places like Himachal Pradesh, Uttarkhand, North-East India, etc
This depends on what you want to explore, your budget, number of days & how far you are willing to go. Assuming you are willing to travel in India, below are the places explored by me that can be considered good for solo travelling excluding North East. In North India, you can plan for Jaipur(history, tradition, monuments; Luxury / Backpacking); Udaipur(Luxury / Backpacking, sightseeing, monuments, history, food); Pushkar(religious, traditional handicrafts/clothes, relaxing, backpacking); Jodhpur(history, monuments, luxury / backpacing, traditional exploration, food); Jaisalmer(Camping, History, safari, Backpacking / Luxury); Agra+Fatehpur sikri / Jhansi+Gwalior(monuments, luxury / backpacking, history), Delhi(Shopping, Food, party, backpacking, monuments, history); Rishikesh / Dehradun / Mussoorie(Relaxing, backpacking, nature, sightseeing, adventure, riligious); Kasol / Mandi / Shimla( adventure, relaxing, sightseeing, backpacking). If in South India, you can plan to explore Pondicherry(Beach, history, party, relaxing, backpacking); Madhurai+Rameshwaram / Kanyakumari(Religious, Beach, backpacking); Mysore(history, backpacking, sightseeing, camping); Udupi(relaxing, backpacking, beach, adventure); Kochi(city exploration, backpacking); Varkala / Kovalam( Beach, party, relaxing, landscapes, backpacking); Hampi(history, backpacking); Sakleshpur+Bellur+Halebaidu(History, backpacking, camping, nature); Gokarna + Karwar(relaxing / adventure; beach / trek, party, backpacking / luxury For central India, plan visit to Indore+Ujjain+Omkareshwar(Religious, streetfood, backpacking); Jabalpur / Bhopal / Khajuraho( waterfall, lake, religious, history, backpacking); Srisailam(religous; sightseing, backpacking), Belum caves + Gandikota (sightseeing; backpacking; cave exploration) Western India, Mumbai / Pune (city exploration, food, nightlife, monuments, backpacking); Mahabaleshwar / Panchgani (hill station, relaxing, backpacking); Goa(relaxing / adventure, backpacking/luxury; party, food, history); Lonavala / Lavasa (relaxing, sighseeing, backpacking); For Northern eastern coast, Puri / Bhuvaneshwar(backpacking, beach, religious); Araku (hills, relaxing, adventure); Kolkata(city exploration, food; traditional clothes; monuments, history); You can contact me for more information on any of the above place.