The biggest part of weight training is motivation. You seem to have it now - don’t give up. Set LONG TERM goals and dont expect immediate results. In fact, it is likely you will GAIN weight before losing it.
Reason: your body will want more water (you will sweat salt so your body will crave more salt, salt absorbs water and retains it - also muscles heal using water) - you will gain water weight. Also, as you put on muscle keep in mind muscles is heavier than fat - so unless you are doing other activites (cardio!!!) you won’t see results quickly.
A personal trainer may be out of the question - what about a work out buddy? Even if you ‘meet’ someone at the gym - maybe not someone who will spot or even motivate you - but knowing someone is going to be expecting you there will keep you going.
Really consider the reasons why you are starting a weight training program - your diet pattern changing is a good thing - are there other reasons? If you are doing it for yourself so you feel better that is a good thing - as opposed to doing it so you think someone else has a better opinion of you.
Tell EVERYONE YOU KNOW what you are doing - that way they will ask you how you are progressing each time you see them - it helps to motivate you (so you can tell them about your improvements in ability to lift heavier weights or your loss of body fat).
Don’t always focus on one muscle group (i.e. chest and arms). Rotate your workouts between upper and lower body, arms, legs, tricepts, bicepts, upper back, lower back, calves, wrists, forearms (don’t forget forearms, if you always do chect and arms and don;t strengthen your wrists and forearms, your other muscles will peak because your forearms and wrists can’t support heavier weights).
Set incremental goals! Measure them! Achieve the incremental goals (which will make you feel proud and like you have accomplished something). Challenge yourself! Never give up! Keep track of where you are now, where you want to be tomorrow and where you hope to be next week. Write it down on paper each day. Write what you eat so you see how it impacts what you are doing.
Celebrate! When you hit an incremental goal, reward yourself.
Do something extra - one Saturday, add something you haven’t done in the past. Do something new.
About every three to four weeks, completely change what you are doing (i.e. if you are doing three sets comprised of a warm up set, medium weight, target weight, change it - do two sets, warm up and burn out; lower the heavy weioght and do more reps; do power sets, etc.). It’s important to chnage how you are workiung out so your muscles don’t become accustomed to what you are doing. Surprise them!!
Keep your doctor informed of what you are doing and how it makes you feel.
Anyway - the fact you’ve started is the biggest success!!! Good luck!!!