Making friends in a hostel is usually easy but not always. Sometimes the layout of the hostel makes all the difference. I tend to research a little on my travels and try to see where most of the solo travelers are going. It is much harder to make friends when everyone else came together. Some hostels have size limits on groups, these tend to be good hostels. The ones with age limits obviously have more young people, so if you are within that age limit there is a higher likelihood you will make a friend.
When I first arrive at a hostel, I go straight to my room to set my stuff down. Depending on the time of day you may be able to chat up people in the room. If it is in the middle of the day, there are normally not many people around. If anyone is there it is a good opportunity to chat them up and figure out a little bit of the area and hostel. Usually a good opening line is what is your name and where are you from.
At breakfast time if the hostel offers breakfast (Free or not), I usually go. Even if it is a little more expensive than I would like it is usually worth it to make a friend to hang out with. Often, I have seen that even people in groups may go down and eat breakfast alone. So, this could be an easier way to integrate yourself into a group. My technique is to go to breakfast and scan the room, depending on the size I may walk around a lap. I look for someone sitting alone and just ask if it is OK if I sit down. Then I start a conversation. Normally If it is a busy hostel, I make a friend.
On the street it is quite difficult. You will likely have people ignore you are who just are not helpful. It is best to just walk up to someone who looks friendly and ask a question. Smile when you walk, if someone smiles back go and speak to them. The worst that is going to happen is that they are rude or don’t won’t to talk. If so keep trying, you will probably never see these people again unless you want to. I after asking a question ask where they are from or what they do. If they keep wanting to talk offer to buy them a beer/coffee/food. If they accept you probably have made a new friend.
The hostel bar is also a great place to meet people. I always try to go. If there is a happy hour this is the time to go. A lot of fellow travelers are on a budget. Even if you are not drinking this is when the most people tend to be there. Alcohol tends to make people more social also. So even if you are bad at making friends go anyways. Usually people will start a conversation with you. If you are uncomfortable starting the conversation this is the time to get out of your comfort zone. If you are awkward so what. You won’t see them again unless you both choose to. It is also a good time to find out what to do the next day. Usually people will tell you what they did that day. If not ask.
Smoking areas tend to be a good spot to chat to people. This only really applies if it is a designated smoking area. Even if you don’t smoke it is a good opportunity to speak to people. You don’t have to smoke for this. If they ask to tell the truth, you are just trying to make some new friends.
Lastly try the common area. There are usually people in here at all hours of the day and night. It is also usually near reception. Talk to people especially when the first arrive.
For me breakfast time seems to be the best spot to meet friends because most of the solo travelers are looking for someone to enjoy the city with. You must go and speak to strangers which is difficult for many, but if you can make yourself do this it is going to be worth it. The second best is going to be the hostel bar. Either way keep trying, it gets easier with practice.