A lot of countries call bars "measures" (that might help with some confusion - in the UK they're bars!)
If you were to have a tie, than there couldn't be any other notes between the start and the end. (Here there is a C between the Ds) Currently what you have is a slur , a curved line across different notes which means to play them smoothly joined (legato).
I was meaning the fourth one, here:
The last two notes sound a little odd coming after a large jump - but this is personal preference!
Probably easiest if I add some more pictures!
Just now:
There's a large jump between the second-last note and the chord after it which can sound a little awkward.
My proposed solution (maybe this isn't needed, but I'll suggest it anyway):
The top stave (staff) here would be taken by the right hand, the bottom the left hand. See how I've moved the low D into the left hand, then put a C after it to sound at the same time as the chord? This means that scale line flows well and the chord can sit on top of the bass note. This was what I was trying to put across with that convoluted writing! And no need to apologise, you're just starting to learn!
So, the two numbers you have on the music at the very start (4/4 for you), are called the time signature and tell the musician how long each bar is. (There are many different types of time signature, and you can change during a piece if you want - I'll link some theory resources below!)
The top number of any time signature tells you how many beats there are in each bar, and the bottom number tells you the "value" of each beat. You have four quarter notes in each bar (crotchets in the UK) - 4/4 !
Some different time signatures:
The top number tells you the number of beats (here all my notes are quarter notes, bottom number is 4)
A whole note is worth 4 quarter note beats, so you can only have one in a bar (measure) of 4/4.
Theory resources:
https://www.musictheory.net/lessons - This site starts right at the beginning and contains its own exercises! It also goes up to fairly advanced theory but the "basics" section should cover what you'll need to know to begin composing.
Some books: https://www.amazon.co.uk/s?k=music+theory&sprefix=music+theory&ref=nb_sb_ss_ts-doa-p_1_12
The ABRSM "Discovering Music Theory" series is pretty good. Also try "Music Theory for Dummies" for a little more humorous approach! From there, "Music Composition for Dummies" can help you improve (speaking from experience)
If any other forum member wants to add to this list, please do!
aMC