coming up with a plan

Thats a really vague question... At your level the best plan is to make sure you defend all your pieces and keep your king safe. Games at the >1000 level are almost always decided by hanging pawns or pieces

If you want advice on specific openings, that's something I can actually help you with

Plans don't really happen below 1500 or 1600 FIDE  

Not that players below that can't plan, in fact sometimes they make very good plans, but the plans never fully happen because one or both players miss a tactic that changes everything. A 10 move plan (for example) can't actually help you if you lose a knight on move 3.

IMO your "plan" should be to try to follow the basic flow of the game, which goes something like this:

Opening:
Bring pieces out quickly and castle the king to safety

Middlegame:
If you can't really attack anything yet, pick one of your opponent's center pawns that can't move forwards and advance one of your pawns to attack it. After the pawn trade happens it will open files and diagonals for the pieces you developed during the opening.

Middlegame (2):
Use the open files and diagonals to infiltrate with your pieces into your opponent's side of the board and / or come into contact with weak pawns (like isolated pawns, or any pawn on its original square) or attack the squares around their king.

This is the basic flow of a chess game. If you can do this well while not missing basic tactics then you'll be ready to learn more detailed ideas.

#1
 "I still don't know how to make a plan"
++ The main thing is to take time to think. Time is a resource, use it.
As a general rule you want to move your piece that is standing the worst.
Think about the position. What would be ideal squares for your pieces? What would be ideal squares for your opponent's pieces? How can you approach your ideal and hinder your opponent's ideal?