Citizens from most of the countries don’t need to obtain a tourist visa before traveling to Nepal. Of course you can get one earlier on, but It’s not a problem if you won’t, as it is available on arrival at Tribhuvan International Airport and at some land borders, depending on your traveling plans. I got mine at the airport, though the queue was very long (2h to wait), and I was really tired after a long fly, I still wouldn’t get in to trouble of getting one before in embassy. The requirements are: valid passport for at least 6 months when submitting your application, fee to pay and 2 passport-size photos. You can pay in pounds, euros, dollars or Nepali rupees at the airport. 
I`ve heard that there are way more currencies you can use to purchase one, but on a safe side better to have one of the main 4, I mentioned. Try to make it as smooth as possible. Trust me, you will be dying to lay on hotel`s bed asap, not to be stack at the airport. Prices for tourist visas are: 25.00$ for 15 days single/multiple entry; 40.00$ for 30 days single/multiple entry and 100.00$ for 90 days single/multiple entry. If you wish to stay for more than 60 days, you can extend your stay up to 30 days by applying to the Nepalese Department of Immigration at Kalikasthan in the capital, or in immigration office in Pokhara. It is very important to have a valid visa in your passport to be able to exit Nepal. If your visa will expired, you will have to arrange an extension at the Department of Immigration before your departure. Regarding the photos, you should bring two passport-sized photos with you (some pages say just one, and that came to be a true in my situation). Well, I was prepared and I’ve had them, but I witnessed loads of people who forgot to bring them. They were allowed to enter anyway, but It might be also due to the fact that the photo machine was broken at the time I was there. Probably is still broken till now.
Packing. It really depend on activities you’re planning to do in Nepal. In Kathmandu Valley is usually very hot, as the area is not that elevated, so I would suggest to pack loads of summer stuff. Make sure to have loads of space, as it is highly possible that you will buy loads of clothes there too. But if you are planning to do some hiking, you definitely need a very warm waterproof clothes and a good hiking shoes. Very good backpack is essential for trekking and all long-term trips in general. Get also a rain cover to protect your stuff in. Check information about monsoon season (June – September), as it can help you to prepare better. Also this time of a year can be hazardous in the rural areas, especially in western Nepal. Monsoon rains cause flooding and landslides that can make your journey difficult and can make some areas unable to reach. Take care and check access routes before. Most of the last base camps are at more than 4.000 m high, so the weather there can be unpredictable. I am not a trekker, so I can’t share many tips with you, but there are loads of good blogs about it. Always use sun block (SPF20 or higher) and sun glasses.
Hotel. There are loads of places to stay in Nepal, as it is a backpackers mecca. You don’t have to book before arriving, but that can help you to save the time looking around for a place you like most. They are all very affordable and comfortable, usually with the breakfast included. I paid for mine around 6-7£ per night, and that was with private bathroom. I remember only in Nagarkot I paid 20£ per night, but I wanted a hotel with the very best view to be able to spot Mt Everest from there.
I consider Nepal, and Asia in general, as a very safe place. There’s a a very low rate of serious crime in Nepal. Of course, like everywhere else, you should always be careful, and you should watch out for pick-pocketing and bag thieving in buses, hotels, hostels and in all touristic areas like Thamel, Sanepa and Kupondol in Kathmandu. They are really overcrowded which makes it easy to still from you. As usual take care when walking around at night after midnight. Though, main ares are always busy, I think it’s better not to wonder around alone. Robberies are more likely to occur in the evening in poorly areas. Don’t carry much of cash with you. Keep valuables in a hotel safe or locker if possible.
Solo female traveller 