Knowing music theory is not really that crucial for composing, experience with what ideas works together is though, which is why I would advise you to start composing at once.
You can't read in a book how to make a great composition, you can find useful advise though, and knowing some theory will be a great supportive tool.
Not having a particular good ear is of course not going to make composing easier for you, but you will not get any better at composing before you actually start to do it and get some real experience with what works and what doesn't.
Just don't give up, you should probably not expect to compose anything worthwhile the first long while, but everything you do compose, no matter the actual quality of it will gradually get you closer, give you a better idea and sense of what works and what doesn't and more and more useful experience for each composition.
Also you might want to experiment with composing using a computer program to write music (programming music as it is usually called, though it has little to do with traditional computer programming).
The reason why I suggest this is that I at least find you get a much better structural overview and general clearer picture and idea of what you are actually doing, plus you will be able to move around with individual sections of the composition and make whatever changes you might wish easily as you go along and play them back instantly to check out if what you did works.
This will make experimenting and exploring your ideas and possibilities in a trial and error kind of way that all new creative challenges kind of require a pretty huge deal easier.
Also using this approach will make you develop a more visual and graphical sense of your compositions which might be in your favor since your ear is not the best.
All DAWs(*1) has a midi sequencer(*2) build in and it is possible to find fairly decent free VST(*3) instruments for download on the internet.
(*1)-(Digital Audio Workstation, a music production tool that can be used both to write, record and edit music)
(*2)-(A grid with a piano roll on the side for plotting in your notes as bars (not bars as in the sense of sections of 4/4 or whatever, but small graphical bars representing notes) which you can adjust to be as long or as short you want for setting the duration of the individual notes, as well as you also will have the option of choosing the exact volume at which each individual note is played back)
(*3)-(A specific type of small app like programs that can be run from a DAW, either emulating an effect machine or emulating an instrument using samples that can be programmed via the midi sequencer or the virtual music sheet build into the DAW).
If you feel like investing in a proper VST sampler that got all the instruments you need, including drums and whole drum sets, with samples in an outstanding quality non of the free VST instruments can really match, I would recommend Native Intruments Kontakt 5.
The stand alone, non VST music production program Reason might also be an option to consider, though I find it more suited for composing electronic music.
As DAW, if you do not already own and use one, I can warmly recommend Reaper, which is free but has all the features that you will need and find in more expensive DAWs.
Using Reaper and VST instruments as your platform for composing you get both the option for writing in standard notation via the build in virtual music sheet or using the midi sequencer to program the music.
No matter if you chose to write your compositions using standard notation or you use the sequencer you will be able to see how it looks in either format, as well as you can also chose to swap between both methods while composing.
Further more you will be able spice up the instrument samples by applying VST effects to your individual instrument tracks.
