If you are ahead in development and have your king castled to safety, try to open up the center with a pawn break to open lines for an attack. In such a situation it is often not a bad idea to sacrifice a pawn in order to open more lines for attacking opportunities.
I am new, and confused.

I'm not an advanced player by any means, but I'm at least 1350 rapid and I have just finished a course about making plans in the middle game, so I should be able to give you some valuable information. Once your have passed the opening stage and you have developed your pieces, you need to come up with a plans. A plan can be long-term, like gaining bishop pair or launching an attack, or it can be short-term like improving the scope of a piece, putting your knights on good outpost spares, blocking the scope of your opponents' pieces, taking advantage of open files, or making trades that damage your opponents' pawn structure in a way that is beneficial to you. So basically you are constantly looking for little 2-3 move plans that will improve your position slightly, and eventually if you do this right you will be in a much better position than your opponent and you will have more tatical opportunities and in turn, win the game.
"I am confused" is used when you don't understand something fully. "I am confusing" means that what you are saying or doing is confusing to other people.

You should learn most important principles of a chess game. That will give you a solid understanding of a chess game and you will know what to do in every position. Then, you should optimize your calculation so you don't miss tactics in your games. Your rating will skyrocket. I can help you with all of this. Message me if you are interested.
@1
"safely developing my pieces and even gaining a few moves of development"
++ That is good. Besides quantitative development, there is also quality of development notably influence on the center. E.g. Bc4 controls central square d5 in 1 move, while g3 and Bg2 controls 2 central squares e4-d5 in 2 moves.
"what to do once I’ve developed those pieces?" ++ Play your worst standing piece.
"end up doing nothing" ++ Each move should accomplish something.
"waiting for the opponent to mess up"
++ That is a viable strategy, but you can make it easier for him to mess up.
"how to develop an attack" ++ You can only attack if your position has some superiority,
e.g. if your opponent has neglected the safety of his king and created weaknesses.
"I’m 700 currently." ++ 700 is a sign of frequent blunders.
Always check your intended move is no blunder before you play it.
Was there any reason to create a brand new account just to start this topic?
Will you please explain how you know, or assume that a second account was made?

Was there any reason to create a brand new account just to start this topic?
Will you please explain how you know, or assume that a second account was made?
Well, the account is new, created a few hours ago, without any games, so it stands to reason that 700 rating (mentioned in the first post) comes from somewhere else.
@nkiristic - Thank you for your reply, which makes perfect sense. This of course means several things. This is either the second account on this site, or his/her 700 rating is from another chess site, and this the first and sole account on this site. If the latter, then the poster may have joined this site to play, but wishes to improve before playing his/her first game here, thus the question.
Interesting - thanks again
@jeaju99 - I have heard often enough from my betters, that when one is unable to see or find a move, to simply rearrange ones pieces. Rooks on open/semi opened files, moves which eliminates a piece doing double duty in protection, or to bolster support/apply pressure, where applicable.

@nkiristic - Thank you for your reply, which makes perfect sense. This of course means several things. This is either the second account on this site, or his/her 700 rating is from another chess site, and this the first and sole account on this site. If the latter, then the poster may have joined this site to play, but wishes to improve before playing his/her first game here, thus the question.
Interesting - thanks again
You're welcome. As for improving without playing games, it is a bit difficult. And it is not easy to give concrete tips if we don's see someone's games. We can give some general tips, but some may not be the best for OP.
@OP
But in general, play longer games, if you have the time. If not 15|10 at the minimum. And most importantly, use that time, don't play too fast. At 700 you are probably making a lot of mistakes and drop pieces. By slowing down, this should improve.
Look through games afterwards, and at the very least, identify blunders and why they happened. If you can understand more, so much the better.
You say that you follow opening principles by developing fast? If that is the case, that is pretty good. What next? Well, it depends on a position. In general, if the opponent hasn't develop as fast as you, looking for pawn breaks, especially in the center if the opponent hasn't castle might be a good idea. If not, then improve some of already developed pieces further if possible, or if you see a possible good move for the opponent, try to stop it, if you feel it is an inconvenience to allow it.
All of this requires experience, so you will need to strengthen your chess understanding.
Along with that, you should try to work on tactics. There are tactics trainers online, chess.com has one. The other is on chess tempo for instance, and there are others. Do 5-10 every day, but slowly, carefully, try to figure them out, try not to guess the move blindly.
Hopefully some of that will be helpful.
@nklistk - "You're welcome. As for improving without playing games, it is a bit difficult. And it is not easy to give concrete tips if we don's see someone's games. We can give some general tips, but some may not be the best for OP. "
Agreed with above. About improving without playing, yes. However the posters question, while possible, is not typical to someone whom has not played. That, along with his/her rating is indicative that that the person is playing
Thank you for replying, and elaborating.

Improving Your Chess - Resources for Beginners and Beyond.....
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell
By the day's end, your customers simply need to hand their issues over to you and would prefer not to consider it. MyAtriumHealth
I find I do a pretty good job of safely developing my pieces and even gaining a few moves of development due to their poor choices and due to me pushing them back while simultaneously bringing out a piece. The problem I have is what to do once I’ve developed those pieces? Now what? I find I actually just end up doing nothing and waiting for the opponent to mess up, because I have no idea how to develop an attack. Any advice from more advanced players? I’m 700 currently. Thanks