Namaste and welcome to the forum!
You should start with Hindi first, it is the bigger of the two languages and you will have a lot more available to learn with it. Plus after you learn Hindi you will be able to read the Nepali script already. The languages are quite similar but there are still many differences and in general a Nepali and Hindi speaker could not understand each other. I read Nepali news sometimes and can make some of it out, I would say say it might be like like the romance languages. But knowing Hindi first would make learning Nepali very easy they share a lot of vocab and grammar. Also Nepali's who work in touristy areas of Nepal often know Hindi. Here are some examples of Hindi vs. Nepali:
तपाईंलाई कस्तो छ?
(tapaailai kasto cha?)
How are you? in Nepali
आप कैसे हैं?
(aap kaise hain?)
How are you in Hindi
मेरो नाम ... हो
(mero naam... ho) My name is in Nepali
मेरा नाम.... है
(Mera naam... hai) my name is in Hindi.
मलाइ सन्चै छ
(malai sanchai cha.) I am good in Nepali.
मैं अच्छा हूँ
(main accha hoon) I am good in Hindi.
Nepali speakers tend to understand Hindi much better than Hindi speakers understand Nepali, this is because a lot of Nepali speakers are exposed to Hindi media from an early age.