Yeah, just me and the chap from the opening section who starts tagging along (I'm not having a terribly good day with names.) I'm not really confident enough to try to join a group so I'll be staying solo. I also suspect that the lack of a manual hasn't helped, I have only a tenuous grasp of the game's features, I feel.
Soloing the flashpoints is pretty much impossible, and likewise for 4 person heroics. Like CptJoker said, 2 person heroics
can be soloed if you've reached a high enough level. I've soloed a couple of them with my Sith Sorcerer. That character is a healer, and my follower, Khem Val, is a tank, so I'm generally able to get through 2 person heroics using crowd control abilities and heals to keep Khem alive. Mind you, even 2 person heroics were designed to be played by 2 players AND their NPC companions.
In my experience, you can solo through the class quests well enough, and get enough experience from sidequests. I suspect there might have been some sidequests you missed somewhere along the line.
If you want a bit more XP, make sure you park your character in a cantina or other rest area before you quit playing. You will get a rested XP bonus the next time you play.
If you're nervous about grouping but interested in giving it a try, I'd recommend looking for a friendly guild on your server to join. Tell them you are new to these types of games, and you might be able to find some of them willing to group with you and give you some tips. They might also have a vent or mumble server allowing for voice chat. Voice chat makes grouping a lot easier, because you don't have to switch between playing the game and chatting. A friendly guild can make the social game a lot easier to get into.
One other thing you definitely need to understand before entering a group, and which I feel this game does a terrible job of explaining (in fact, I don't believe it explains it at all), is the "Need Before Greed" system for group loot. I've seen a fair number of new players using this system wrong, and pissing off other people in a group. When you are in a group, and a piece of equipment is dropped, each member of the group will be prompted to select "Need" or "Greed" for it. A random number roll will determine which player gets the item, but players who select "Need" will have priority over those who select "Greed". If it's a piece of equipment intended for your character's class and is better than what you currently have equipped, then select "Need". Otherwise, select "Greed".
Never be afraid to select "Greed", but only select "Need" when it's truly something your character needs to improve his/her stats. For instance, Smuggler equipment imroves Cunning. If a piece of light armor drops that imrpoves Willpower, your character might technically be able to equip it, but the item was clearly meant for a Consular or Inquisitor. Always select "Greed" for equipment designed for other classes, or for miscellaneous items like Stims. Some new players select "Need" for everything, and end up angering other players and getting a really negative experience out of it. So, when in doubt, select "Greed".
CptJoker mentioned Crowd Control abilities. These are very helpful in Flashpoints and 4 person heroics, which were clearly designed with the intention of players disabling enemies to reduce the number they had to fight simultaneously, but they can even help save your life when playing solo. Because causing damage to a crowd-controlled enemy will end the effect prematurely and put that enemy back into the fight, it is important for groups to communicate effectivly which enemies to disable and which to attack. Be careful using area of effect attacks in this situation.
Smugglers have the "Slice Droid" ability which can be used to disable a droid for 60 seconds. If you are in a group, you can offer to use this ability and your group will be thankful. When you approach a group of enemies with one or more droids, offer to CC one of them. For instance, say "I'll cc the droid on the far right" or something. This will let your teammates know not to attack that enemy. At the start of the fight, use slice droid on that enemy, then start attacking one of the other enemies (preferably the one the other members of your group are focusing on). Multiple members of your group using their CC abilities effectively can make these fights much, much easier.
Another tip I would give (and this one took me frightfully long to figure out) is to tweak your keyboard mapping a lot to make use of as much of your keyboard as possible, so you don't have to rely on pressing quickbar buttons with your mouse. If you set the UI to show additional quickbars on the sides or bottom, you can map whatever keys you want to the buttons on those bars. Work on setting up your key mappings in a way that works for you personally. If there are certain keys you find yourself never using, you might as well remap them for something you would use. For instance, I never used the Q and E keys for strafing. I would strafe by holding the right mouse button and pressing A or D. So I remapped Q and E for abilities I need frequently. Q activates my ability which breaks me out of a hold, while E will use my interrupt ability. The function keys at the top of the keyboard are also ripe for remapping. Don't let conveniently placed keys go to waste.