It might be worth as a company investing in some recruitment training, you may be able to buy a package online for not much cost and could avoid expensive mistakes? In general though here are my top tips:
Check if candidates have any disabilities requiring adjustments beforehand, e.g. sight loss meaning they need any written resources in a specific format, hearing loss meaning they need certain audio adjustments. It's no good learning about this on the day of the interview!
Always interview in at least pairs, never alone, I know this can be hard with limited resources but (a) it's invaluable to have a second opinion, can really help limit the impact of any personal hang-ups, biases or prejudices and (b) in the worst case scenario of some kind of complaint, gives you back-up of what was said and done.
Prepare your questions in advance and stick to the same list for each candidate, you can ask tailored follow-ups but stick to the same basic structure so you can compare candidates afterwards.
Always ask open questions not closed ones, so e.g. 'tell us about your experience in sales to high end clients' not 'have you worked with high end clients?'.
Ask questions in a way which prompts candidates to give examples and evidence of what they've done, not to answer in generalities or hypotheticals. So 'tell me about a time when you've dealt with a difficult customer' not 'how would you deal with a difficult customer'. If they do answer in a vague/general way, say 'that's great, can you give me an example of when you put that into practice?'. This goes some way (although not fully) to identifying the bullshit merchants who can talk the talk but have never actually done anything!
Always do a welcome opening schpiel introducing yourself and your co-interviewer, explaining what will happen, who will talk etc. and then ask an easy warm up question like 'tell us why you've applied for the role' - gives candidates time to settle in and warm up before the harder questions later.
Keep your own body language and tone nice and open and warm, candidates should then mimic you - if you mumble, hunch, look at your keyboard rather than at them etc. candidates will often feel you are disinterested and respond accordingly.
Don't be afraid to use tests, written exercises, presentations (if relevant for the role) etc as well as interviews, interviewing well is a real skill and not one everyone has, using a broader range of techniques can get you the best candidates including those you might miss because they are a nervous or reticent interviewee. For tests I often use (edited/anonymised) examples of real life work they'd have to do, e.g. here's an email from a client, you've got 15 minutes to draft a response (obviously not anything that requires specialist knowledge of your company/working practices), here's some stats on an excel sheet, you have 30 mins to draft this into a presentation format, here's 10 tasks, you have 10 mins to prioritise them and tell me which you'd handle first and why, that kind of thing, it doesn't have to be super complicated....
Good luck!