The first song just needs to fit in there, try a song you feel can be pretty good as a starter and listen to your mixtape afterwards. What kind of feel does the first song give you? Make a couple of test mixtapes with different arranged starters and see what excites YOU the most.
As a DJ, you can't always be a delivering machine of what people like or doesn't, everybody is different and you can't please all, so it's important at this point to do what YOU actually like and feel should be shared with the rest of world / make them feel just as good. If you don't have confidence in your own taste of music, neither will other people. That's why there are 50 different DJ's at every event, instead of just 1 that plays whatever is popular.
There are different ways of transitioning, not all songs have to be intro > outro, so it always depends on what kind of music that you have and be sure to line them up accordingly to what you feel that they will fit perfectly in transition.
Bad transitions are not always that the actual DJ is bad, it's more his arrangements of tracks is careless, when you try to transition two songs that doesn't quite fit with eachother, it'll always sound bad, no matter how skilled you are. So keep it flowing, have a great taste in music and generally stay away from all sort of unnecesary crap that people doesn't need to hear.
They want to listen to music, they want to dance and have a great time, they are not there to be impressed by your DJ'ing skills, but it is your job to make sure they get that smoothly with no dirty tricks.
To spice things up, I'd say it's all in what kind of songs you choose. Mashups are amazing at spicing things up and making your set unique.
In the end, practice a lot, try out new things, listen to your own mixtapes, listen to other DJ's mixtapes, what do they do wrong in your opinion? what could they have done better, what are the things that bothered you? what did you like about a certain mixtape?
Take inspiration from others and build on it to make it better, give to people what you think they are missing out on.
The good thing with DJ'ing is, you are free to do whatever you want to without having to share it with anybody, so get that booth up and running and try EVERYTHING, bad or good, it doesn't matter, just so you can filter out the things that you hate the most and achieve the style that you like.
DJ'ing is a lot about YOU.. I can't sit here too much telling you about what certain things are wrong or right, it's all for you to find out for your own style.